Add ALEStartLint autocmd

This grew out of my work in #1193; to ensure the statusline was being
updated I had to add:

    fun! s:redraw(timer)
        redrawstatus
    endfun

    augroup ALEProgress
        autocmd!
        autocmd BufWritePost * call timer_start(100, function('s:redraw'))
        autocmd User ALELint redrawstatus
    augroup end

Which kind of works, but is ugly. With this, I can replace the
`BufWritePost` with:

    autocmd User ALEStartLint redrawstatus

Which is much better, IMHO.

Actually, this patch actually replaces adding a function, since you can
do:

    augroup ALEProgress
        autocmd!
        autocmd User ALEStartLint hi Statusline ctermfg=darkgrey
        autocmd User ALELint      hi Statusline ctermfg=NONE
    augroup end

or:

    let s:ale_running = 0
    let l:stl .= '%{s:ale_running ? "[linting]" : ""}'
    augroup ALEProgress
        autocmd!
        autocmd User ALEStartLint let s:ale_running = 1 | redrawstatus
        autocmd User ALELint      let s:ale_running = 0 | redrawstatus
    augroup end

Both seem to work very well in my testing.

No need to `ale#Statusline#IsRunning()` anymore, I think?

README.md

@@ -40,7 +40,7 @@ formatting tools, and some Language Server Protocol and `tsserver` features.
     5. [How can I show errors or warnings in my statusline?](#faq-statusline)
     6. [How can I show errors or warnings in my lightline?](#faq-lightline)
     7. [How can I change the format for echo messages?](#faq-echo-format)
-    8. [How can I execute some code when ALE stops linting?](#faq-autocmd)
+    8. [How can I execute some code when ALE starts or stops linting?](#faq-autocmd)
     9. [How can I navigate between errors quickly?](#faq-navigation)
     10. [How can I run linters only when I save files?](#faq-lint-on-save)
     11. [How can I use the quickfix list instead of the loclist?](#faq-quickfix)
@@ -493,15 +493,17 @@ Will give you:
 
 <a name="faq-autocmd"></a>
 
-### 5.viii. How can I execute some code when ALE stops linting?
+### 5.viii. How can I execute some code when ALE starts or stops linting?
 
 ALE runs its own [autocmd](http://vimdoc.sourceforge.net/htmldoc/autocmd.html)
-event whenever has a linter has been successfully executed and processed. This
-autocmd event can be used to call arbitrary functions after ALE stops linting.
+events whenever has a linter is started and has been successfully executed and
+processed. This autocmd event can be used to call arbitrary functions before and
+after ALE stops linting.
 
 ```vim
 augroup YourGroup
     autocmd!
+    autocmd User ALEStartLint call YourFunction()
     autocmd User ALELint call YourFunction()
 augroup END
 ```

autoload/ale/engine.vim

@@ -556,6 +556,8 @@ function! s:RunJob(options) abort
         \   'output': [],
         \   'next_chain_index': l:next_chain_index,
         \}
+
+        silent doautocmd <nomodeline> User ALEStartLint
     endif
 
     if g:ale_history_enabled

doc/ale.txt

@@ -2196,6 +2196,12 @@ ALELint                                                       *ALELint-autocmd*
   The autocmd commands are run with |:silent|, so |:unsilent| is required for
   echoing messges.
 
+
+ALEStartLint                                             *ALEStartLint-autocmd*
+
+  This |User| autocommand is triggered by ALE right after it started a new
+  linting job.
+
 ===============================================================================
 10. Special Thanks                                         *ale-special-thanks*
 

test/test_alelint_autocmd.vader

@@ -1,18 +1,41 @@
 Before:
+  let g:start = 0
   let g:success = 0
   let g:ale_run_synchronously = 1
 
+  function! TestCallback(buffer, output)
+    return [{
+    \ 'lnum': 1,
+    \ 'col': 3,
+    \ 'text': 'baz boz',
+    \}]
+  endfunction
+
+  call ale#linter#Define('foobar', {
+  \ 'name': 'testlinter',
+  \ 'callback': 'TestCallback',
+  \ 'executable': has('win32') ? 'cmd' : 'true',
+  \ 'command': has('win32') ? 'echo' : 'true',
+  \})
+  "let g:ale_linters = {'foobar': ['lint_file_linter']}
+
 After:
   let g:ale_run_synchronously = 0
   let g:ale_buffer_info = {}
+  let g:ale_linters = {}
+  call ale#linter#Reset()
+  delfunction TestCallback
   augroup! VaderTest
 
 Execute (Run a lint cycle, and check that a variable is set in the autocmd):
+  set filetype=foobar
   augroup VaderTest
     autocmd!
-    autocmd User ALELint let g:success = 1
+    autocmd User ALEStartLint let g:start = 1
+    autocmd User ALELint      let g:success = 1
   augroup end
 
   call ale#Lint()
 
+  AssertEqual g:start, 1
   AssertEqual g:success, 1