Add early-parsed conditionals for mobile detection

Hi Rob,

Feel free to reject if you consider this adding bloat to your Mo Variables, it's just that MV is such a default tool for our sites that I always think of it when I need some custom global variables. So thanks for this handy add-on!

As for the pull request: Adds a few simple global variables for mobile detection: `{is_mobile}`, `{is_not_mobile}`, `{is_tablet}`, `{is_not_tablet}`, `{is_phone}`, `{is_not_phone}`

Harnesses the Mobile Detect library: [http://mobiledetect.net/](http://mobiledetect.net/)

Cheers,
John