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.
A followup ticket to make all of this infrastructure nicer is in order. No need to fix it with this ticket, methinks.