Add support for block strings (triple quotes)

As per [GraphqQL RFC](https://facebook.github.io/graphql/draft/#sec-String-Value) and its [implementation](graphql/graphql-spec#327), block strings are being a GraphQL supported feature for a while and are used for GraphiQL documentation. This PR add support for the triple quotes (""") block string.

GraphQL.sublime-syntax

@@ -13,10 +13,18 @@ contexts:
       scope: punctuation.definition.comment.graphql
       push: line_comment
 
+    - match: (""")
+      scope: punctuation.definition.comment.begin.graphql
+      push:
+        - meta_scope: comment.block.graphql
+        - match: (""")
+          scope: punctuation.definition.comment.end.graphql
+          pop: true
+
   main:
     # Strings begin and end with quotes, and use backslashes as an escape
     # character
-    - match: '"'
+    - match: '"\w+'
       scope: punctuation.definition.string.begin.graphql
       push: double_quoted_string