Opening configuration up for extension:

Added a gvimrc file, which only contains a source ~/.gvimrc-ext.
Added a source ~/.vimrc-ext to the .vimrc
Added my gvimrc-ext and vimrc-ext as examples
Renamed files and updated markdown

Changes made based on comments in response to the plull request.
* renamed the *vimrc-ext files to *vimrc-ext-example
* added an extending and overriding section to the markdown
Made the extensions optional.