Add blocklist file to ignore unused addons

[Crandel]

Can we have a blocklist file for unused addons and during installation CSS code from this addons will not be included to result userContent.css file?

I use only uBlock Origin and tridactyl and a lot of custom CSS code and it's really hard to open userContent.css file in Emacs and even in Vim to inspect final result. Also I think there will be performance improvements to load smaller file in browser.

It could be just simple text file with name blocklist in ShadowFox_customization folder
with one addon file name per line from this folder