Removed replace mode from vim config.

* I don't *ever* use it, so there's no point in keeping a semi-correctly
  behaving mode.
  => Mapped r to recompiling current C source file.
  => Need to write script to handle R to compile a project (search Makefile, and
     so on).
* Removed _ from word-separating characters' list; handy, but syntax
  highlighting went crazy.