Add support for linting Handlebars templates with ember-template-lint (#452)

* Ember-template-lint Handlebars template linter: initial handler, test.

* Handlebars support with ember-template-lint: basic documentation entries.

ale_linters/handlebars/embertemplatelint.vim

@@ -0,0 +1,55 @@
+" Author: Adrian Zalewski <aazalewski@hotmail.com>
+" Description: Ember-template-lint for checking Handlebars files
+
+let g:ale_handlebars_embertemplatelint_executable =
+\   get(g:, 'ale_handlebars_embertemplatelint_executable', 'ember-template-lint')
+
+let g:ale_handlebars_embertemplatelint_use_global =
+\   get(g:, 'ale_handlebars_embertemplatelint_use_global', 0)
+
+function! ale_linters#handlebars#embertemplatelint#GetExecutable(buffer) abort
+    if g:ale_handlebars_embertemplatelint_use_global
+        return g:ale_handlebars_embertemplatelint_executable
+    endif
+
+    return ale#util#ResolveLocalPath(
+    \   a:buffer,
+    \   'node_modules/.bin/ember-template-lint',
+    \   g:ale_handlebars_embertemplatelint_executable
+    \)
+endfunction
+
+function! ale_linters#handlebars#embertemplatelint#GetCommand(buffer) abort
+    return ale_linters#handlebars#embertemplatelint#GetExecutable(a:buffer)
+    \   . ' --json %t'
+endfunction
+
+function! ale_linters#handlebars#embertemplatelint#Handle(buffer, lines) abort
+    if len(a:lines) == 0
+      return []
+    end
+
+    let l:output = []
+
+    let l:input_json = json_decode(join(a:lines, ''))
+    let l:file_errors = values(l:input_json)[0]
+
+    for l:error in l:file_errors
+        call add(l:output, {
+        \   'bufnr': a:buffer,
+        \   'lnum': l:error.line,
+        \   'col': l:error.column,
+        \   'text': l:error.rule . ': ' . l:error.message,
+        \   'type': l:error.severity == 1 ? 'W' : 'E',
+        \})
+    endfor
+
+    return l:output
+endfunction
+
+call ale#linter#Define('handlebars', {
+\   'name': 'ember-template-lint',
+\   'executable_callback': 'ale_linters#handlebars#embertemplatelint#GetExecutable',
+\   'command_callback': 'ale_linters#handlebars#embertemplatelint#GetCommand',
+\   'callback': 'ale_linters#handlebars#embertemplatelint#Handle',
+\})