Allow whitespace before name-separator in jsonKeywordMatch/Region.

The current keyword syntax highlighting regex successfully handles the
following json :

  "keyword":"value",
  "keyword": "value",

but does not match:

  "keyword" : "value",
  "keyword" :"value",

This patch permits keyword highlighting to work as intended in these
unsupported cases.

@see http://www.ietf.org/rfc/rfc4627.txt