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.
I believe this will mean that parsing errors will be shown in the popup itself now. That's probably not a good idea. Also, this conflicts with the console.log calls below ("Dump some diagnostics messages, parsed out by the plugin."). Luckily this doesn't break the plugin because of some try/catches in the python file, but that's far from ideal.

I'm not sure if this was in place when you wrote this patch, but you should move this back where it was. Unfortunately, there's currently no good way for dumping diagnostics *after* starting the output. A much nicer way of handling this would probably be to buffer the output into two stores, and dumping them separately via console.log at the end, but I don't think fixing that is a priority now.