Allow setting projectile root as a file local variable.

This is useful, for example, if you keep your OrgMode files in one git repo but associate them with other projects in other directories. It just saves you having to switch to a different project all the time.

This is my first time contributing to a big elisp project so I have no idea if this is following the proper conventions. Any feedback is appreciated! Also I have no idea if anyone else would find this functionality useful, but I just implemented it for myself in my .emacs.d and decided to push it up here too in case it was useful for others.