in_http: config: cors_allow_origins now is a JSON array.

This patch makes the cors_allow_origins configuration key to store
a JSON array, it can be used as follows:

 <source>
   type http
   port 9880
   cors_allow_origins ["http://example.com", "http://www.treasuredata.com"]
 </source>

Signed-off-by: Eduardo Silva <eduardo@monkey.io>