Add ECR (Embedded Crystal) (#2996)

* Add ECR entry from language-crystal

* Add HTML+ECR to Languages.

* Create greeting.erb sample

From the ECR documentation: http://crystal-lang.org/api/ECR.html

* Rename greeting.erb to greeting.ecr

* Rename samples/HTML-ERB/greeting.ecr to samples/HTML-ECR/greeting.ecr

* Update Crystal submodule.

This contains the ECR highlighting.

* Proper named HTML+ECR folder.

The GitHub website just wouldn't do the folder right.

* Add .ecr file extension to ECR

lib/linguist/languages.yml

@@ -1404,6 +1404,16 @@ HTML+Django:
   - html+jinja
   - htmldjango
   ace_mode: django
+  
+HTML+ECR:
+  type: markup
+  tm_scope: text.html.ecr
+  group: HTML
+  aliases:
+  - ecr
+  extensions:
+  - .ecr
+  ace_mode: text
 
 HTML+EEX:
   type: markup