Add ruby fixer using rubocop --auto-correct (#689)

* add ruby fixer for `rubocop --auto-correct`

autoload/ale/fix/registry.vim

@@ -42,6 +42,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['python'],
 \       'description': 'Fix Python files with yapf.',
 \   },
+\   'rubocop': {
+\       'function': 'ale#fixers#rubocop#Fix',
+\       'suggested_filetypes': ['ruby'],
+\       'description': 'Fix ruby files with rubocop --auto-correct.',
+\   },
 \}
 
 " Reset the function registry to the default entries.

autoload/ale/fixers/rubocop.vim

@@ -0,0 +1,31 @@
+" Set this option to change Rubocop options.
+if !exists('g:ale_ruby_rubocop_options')
+    " let g:ale_ruby_rubocop_options = '--lint'
+    let g:ale_ruby_rubocop_options = ''
+endif
+
+if !exists('g:ale_ruby_rubocop_executable')
+    let g:ale_ruby_rubocop_executable = 'rubocop'
+endif
+
+function! ale#fixers#rubocop#GetExecutable(buffer) abort
+    return ale#Var(a:buffer, 'ruby_rubocop_executable')
+endfunction
+
+function! ale#fixers#rubocop#GetCommand(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'ruby_rubocop_executable')
+    let l:exec_args = l:executable =~? 'bundle$'
+    \   ? ' exec rubocop'
+    \   : ''
+
+    return ale#Escape(l:executable) . l:exec_args
+    \   . ' --auto-correct %t'
+
+endfunction
+
+function! ale#fixers#rubocop#Fix(buffer) abort
+    return {
+    \   'command': ale#fixers#rubocop#GetCommand(a:buffer),
+    \   'read_temporary_file': 1,
+    \}
+endfunction