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.
We want to merge the local "global" options with the parent "global" options. This is the way jshint works.
(assigning would replace all the parent global options by the local global options which is not what we want to do.