Handle "extends" syntax in options (JSHint 2.5.1)

Fixes #82
This looks for "extends" in the loaded options; if found, the referred file is parsed too, then used as "base" options to which the current ones are added (overwriting if applies).

NB: would recommend to rename parseJSON in parseOptions.
Nitpick: please try to keep the formatting consistent with the rest of the file. This means that comments are sentences have a space after the //, start with an uppercase letter, and end with a fullstop.