add xo support (#304)

* add xo support

* add documentation

* Fix a screw up when fixing conflicts

* Fix it harder

ale_linters/javascript/xo.vim

@@ -0,0 +1,41 @@
+" Author: Daniel Lupu <lupu.daniel.f@gmail.com>
+" Description: xo for JavaScript files
+
+let g:ale_javascript_xo_executable =
+\   get(g:, 'ale_javascript_xo_executable', 'xo')
+
+let g:ale_javascript_xo_options =
+\   get(g:, 'ale_javascript_xo_options', '')
+
+let g:ale_javascript_xo_use_global =
+\   get(g:, 'ale_javascript_xo_use_global', 0)
+
+function! ale_linters#javascript#xo#GetExecutable(buffer) abort
+    if g:ale_javascript_xo_use_global
+        return g:ale_javascript_xo_executable
+    endif
+
+    return ale#util#ResolveLocalPath(
+    \   a:buffer,
+    \   'node_modules/.bin/xo',
+    \   g:ale_javascript_xo_executable
+    \)
+endfunction
+
+function! ale_linters#javascript#xo#GetCommand(buffer) abort
+    return ale_linters#javascript#xo#GetExecutable(a:buffer)
+    \   . ' ' . g:ale_javascript_xo_options
+    \   . ' --reporter unix --stdin --stdin-filename %s'
+endfunction
+
+function! ale_linters#javascript#xo#Handle(buffer, lines) abort
+    " xo uses eslint and the output format is the same
+    return ale_linters#javascript#eslint#Handle(a:buffer, a:lines)
+endfunction
+
+call ale#linter#Define('javascript', {
+\   'name': 'xo',
+\   'executable_callback': 'ale_linters#javascript#xo#GetExecutable',
+\   'command_callback': 'ale_linters#javascript#xo#GetCommand',
+\   'callback': 'ale_linters#javascript#xo#Handle',
+\})