Add string group to syntax files

This commit adds the string group (constant.string) to valid strings
in all the syntax files that support colorschemes (see the readme in
the runtime/syntax directory).

Fixes #178

runtime/syntax/javascript.micro

@@ -17,4 +17,4 @@ color constant   "\\[0-7][0-7]?[0-7]?|\\x[0-9a-fA-F]+|\\[bfnrt'"\?\\]"
 color comment "(^|[[:space:]])//.*"
 color comment "/\*.+\*/"
 color todo "TODO:?"
-color constant ""(\\.|[^"])*"|'(\\.|[^'])*'"
+color constant.string ""(\\.|[^"])*"|'(\\.|[^'])*'"