Added support for linting of proto files (#1098)

* Added support for linting of proto files
* Added function to get the proper protoc command

ale_linters/proto/protoc_gen_lint.vim

@@ -0,0 +1,19 @@
+" Author: Jeff Willette <jrwillette88@gmail.com>
+" Description: run the protoc-gen-lint plugin for the protoc binary
+
+function! ale_linters#proto#protoc_gen_lint#GetCommand(buffer) abort
+    let l:dirname = expand('#' . a:buffer . ':p:h')
+
+    return 'protoc'
+    \   . ' -I ' . ale#Escape(l:dirname)
+    \   . ' --lint_out=. ' . '%s'
+endfunction
+
+call ale#linter#Define('proto', {
+\   'name': 'protoc-gen-lint',
+\   'lint_file': 1,
+\   'output_stream': 'stderr',
+\   'executable': 'protoc',
+\   'command_callback': 'ale_linters#proto#protoc_gen_lint#GetCommand',
+\   'callback': 'ale#handlers#unix#HandleAsError',
+\})