`force` option, JSON output, DOM element reference

This PR closes #9 as well as brings a bunch of new features:
- `outputFormat` option and JSON format output, default is current text format
- `verbose` output to console, to remove noise from console by default
- `domElement` option to include reference (tag name, class names & id) to reported elements. Optional for both output formats.

Also this PR includes other changes:
- Console output includes not only ERROR and NOTICE messages, but WARNING as well. Thus coloring has been changed.
- Gruntfile moved to plain JS for sake of consistency with entire codebase.

The major addition here is JSON output, it's useful for HTML reports. Also reference to DOM element brings more detailed report, this is a part of HTML reporter. I'm not sure if tag name, class names and id are enough info about the element, it's definitely not. Still searching for another way, maybe some kind of source maps.