Improve reST CSS support about pelican-bootstrap3

[DonDebonair]

I'll make the necessary changes sometime this week, and will test-drive it with both rst docs and md docs on a few different Bootstrap 3 themes to see if nothing breaks :)

[DonDebonair]

I opted to make the inclusion of html4css1.css optional because it's an extra css file to load. A file that's unnecessary if you're using Markdown. See the README for instructions. I also opened an issue with Pelican, because ideally you'd want to be able to do this automatically, based on the Reader (parser) used. See getpelican/pelican#1102

I hope this solution is satisfactory for you :)