Primitive extension system

https://github.com/marienfressinaud/FreshRSS/issues/252
I have been using this extension system for a little while, in
particular to include custom CSS and/or JavaScript (inclusion of PHP
code is not done yet).
There is very little code and it does not impact performances.
I hurry to post it before
https://github.com/marienfressinaud/FreshRSS/issues/655