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?

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