vimrc: set tags redefinition

As usual, generate project wise tags with ctags -R

This mix of options was taken from [1], but I just threw them all
together since I'm lazy and this worked. It's possible I re-implemented
;<path>

  1: http://stackoverflow.com/questions/5017500/vim-difficulty-setting-up-ctags-source-in-subdirectories-dont-see-tags-file-i

Signed-off-by: Carl Helmertz <chelmertz@op5.com>