Added optional bootstrap and font awesome includes

[carl-alberto]

A lot of us know that bootstrap is bad for wordpress based from this article
http://themeshaper.com/2014/08/19/why-bootstrap-is-a-bad-fit-for-wordpress-themes/

But there are still a lot of front end developers that loves to use underscores and bootstrap because it gives us the ability to write shorter code in implementing responsive sites by familiarizing the bootstrap grid system and has greatly increased our responsive implementation by more than two folds.

Hopefully we can include this as an optional enqueue script in benefit for all of those that loves underscores and bootstrap folks out there.

Thanks for sharing us underscores! more power to all of you guys!

[emiluzelac]

There is no true benefit from this, anything beyond what _s already does is themers decision :)

[philiparthurmoore]

This could possibly find its way as an optional part of the download on underscores.me but this doesn't belong in the core _s package.

[grappler]

There is a fork of _s that has Bootstrap integration. https://github.com/Themekraft/_tk/