Improve reST CSS support

[memeplex]

In general, the html4css1.css stylesheet should be included along the rest of the css as documented in this page.

Probably this is at worst harmless, because this stylesheet is mostly concerned with docutils specific classes so it doesn't overlap with more general css theming.

A simple example in which you can appreciate the difference between including and not including this file:

i.  first item
ii. second item

The specified behavior is that the generated list must show the correct (roman) numeration style.

[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 :)