grammar: add (double quoted) string interpolations #{test} using Ruby

Related to adamchainz/fluentd.tmLanguage#3

CHANGELOG.md

@@ -10,6 +10,7 @@ NEW FEATURES:
 - Support array and hash (using JSON source) value types like `["foo","bar"]`
 - Support integer, time and size value types (e.g. `property 100`, `bufsize 5M` or `finterval 5s`)
 - Support highlighting ${variables} (used by e.g. [record_transformer plugin](https://docs.fluentd.org/v0.12/articles/filter_record_transformer))
+- Support (double quoted) string interpolations like `#{Time.new.to_i + 1}` (using Ruby source)
 
 IMPROVEMENTS:
 

grammars/fluentd.cson

@@ -93,6 +93,7 @@ repository:
         name: 'meta.value.string.quoted.double.fluentd'
         patterns: [
           {include: '#stringescapes'}
+          {include: '#stringinterpolations'}
         ]
       }
       {
@@ -168,3 +169,14 @@ repository:
         name: 'constant.string.escape.fluentd'
       }
     ]
+  stringinterpolations:
+    patterns: [
+      {
+        begin: '#\\{'
+        end: '\\}'
+        name: 'constant.string.interpolation.fluentd'
+        patterns: [
+          {include: 'source.ruby'}
+        ]
+      }
+    ]