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.
Should use path.sep instead of "/".

Also, I'm not sure how accurate it is to check for absolute paths this way. On Windows, baseFile[0] could potentially be something entirely unexpected Let's not trust user input and use path.resolve to definitively check this.