Added the possibility to read a custom.css file for tweaking the final html in full_html and reveal templates.

The idea behind this PR is give to the advanced users the possibility to add a custom.css in the same directory where the final html (full_html or reveal) lives to easily changes some specific css.

This PR follow the same idea of live slidemode where you can define a custom.css file (now named main.css).

@fperez also suggest this kind of customization when I explain my PR about customizing things with the notebook UI (https://github.com/ipython/ipython/pull/3432), which I believe is important from the design point of view for some users, but will be released as a extension. 

For example, a use case: 
* to use the darker themes in reveal, you have to add some css modifications... and do it editing by hand the html source code is a pain...

Following the self-campaign of at least one tiny but useful (I think) PR a day... I wait for your comments ;-)

Damián.