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.

doc/ale-handlebars.txt

@@ -0,0 +1,34 @@
+===============================================================================
+ALE Handlebars Integration                             *ale-handlebars-options*
+
+
+-------------------------------------------------------------------------------
+ember-template-lint                          *ale-handlebars-embertemplatelint*
+
+g:ale_handlebars_embertemplatelint_executable
+                              \ *g:ale_handlebars_embertemplatelint_executable*
+
+  Type: |String|
+  Default: `'ember-template-lint'`
+
+  ALE will look for ember-template-lint executable in ancestor node_modules
+  directory. When it cannot find it, this variable will be used instead.
+
+  If you wish to use only a globally installed version of ember-template-lint,
+  set |g:ale_handlebars_embertemplatelint_use_global| to `1`.
+
+
+g:ale_handlebars_embertemplatelint_use_global
+                              \ *g:ale_handlebars_embertemplatelint_use_global*
+
+  Type: |Number|
+  Default: `0`
+
+  This variable controls whether or not ALE will search for a local
+  ember-template-lint executable first. If this variable is set to `1`, then
+  ALE will always use the global version of ember-template-lint, in preference
+  to version installed in local node_modules directory.
+
+
+-------------------------------------------------------------------------------
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale.txt

@@ -31,6 +31,8 @@ CONTENTS                                                         *ale-contents*
       erlc................................|ale-erlang-erlc|
     fortran...............................|ale-fortran-options|
       gcc.................................|ale-fortran-gcc|
+    handlebars............................|ale-handlebars-options|
+      ember-template-lint.................|ale-handlebars-embertemplatelint|
     html..................................|ale-html-options|
       htmlhint............................|ale-html-htmlhint|
       tidy................................|ale-html-tidy|
@@ -123,6 +125,7 @@ The following languages and tools are supported.
 * Fortran: 'gcc'
 * Go: 'gofmt -e', 'go vet', 'golint', 'go build', 'gosimple', 'staticcheck'
 * Haml: 'hamllint'
+* Handlebars: 'ember-template-lint'
 * Haskell: 'ghc', 'hlint'
 * HTML: 'HTMLHint', 'proselint', 'tidy'
 * Java: 'javac'