.vimrc: add vim-addon-local-vimrc

From https://github.com/MarcWeber/vim-addon-local-vimrc:

 Features:
 - When Vim starts up, every directory from root to the directory of the
   file is traversed and special files such as .(local-)vimrc files are
   sourced

 - Because you don't want to run untrusted code by accident, this plugin
   calculates a hash before sourcing. If its unknown you must confirm
   sourcing the file. The hash is updated automatically if you write a
   local vimrc file.

 - if you change a directory and edit a file the local vimrc files are
   resourced