vim: define default filename patterns for vimrc etc

This is meant to match:
 - vimrc, gvimrc (typically used in dotfiles repos or system-wide config)
 - .vimrc, _vimrc (config file name for unix/windows)
 - .gvimrc, _gvimrc (GUI config file name unix/windows)
Merge pull request #120 from blueyed/vim-add-vimrc-pattern

vim: define default filename patterns for vimrc etc