Feature: Add a new task to deploy demo to GitHub Pages

[erikjung]

Re: #70

This adds the Option B solution.

gulp build
gulp demo

The contents of .dist will be committed and pushed to the gh-pages branch, and viewable at http://cloudfour.github.io/drizzle/

NOTE: I first attempted to have the cacheDir (demo) be the same as dist, but the Gulp plugin had issues with that. I might be possible though, for future improvement.

[gerardo-rodriguez]

Awesome! I didn't try it locally, but LGTM! 👍

[tylersticka]

@erikjung You can avoid the demo directory if you just use the gh-pages package (no Gulp plugin). Example: https://github.com/cloudfour/svg-icon-example/blob/master/gulpfile.js#L58-L60

[erikjung]

@tylersticka As mentioned in Slack...

I attempted to use the gh-pages package instead, and experienced some issues with the dist path. I think it was likely just my working copy, but I'm going to keep it as-is for now (using the Gulp package).

We can switch it up later.

[tylersticka]

LGTM 👍