change install location from `~/.vim/` to `~/wptt-vim-config/`

By installing the files in a separate directory, `~/wptt-vim-config/`, and then symlinking all three
- `.vimrc`
- `.gvimrc`
- `.vim/`

This makes it easier to switch between Vim configurations (by changing these three symlinks).

I can see this as more of a preference than a fix so I can understand declining this PR.  However, I've been very impressed with your Vim setup, particularly with your recent addition of Vundle, and I could see this repo becoming a great resource for other WordPress developers who want to get setup using Vim.  I think this change could make it easier for others who want to try out your configuration or switch between multiple configurations.

Thanks for sharing.