Allows to add custom plugins

* Searches *.vim files  on _vundle_plugins_ subfolders. Also excludes them from the repository so they can be added without making the working directory dirty.
* Adds _InstallPlugin_ function that can be used by custom plugins files

For instance, adding __vim-move.vim__ to _vundle_plugins/custom_ will include that plugin:
```
call InstallPlugin('matze/vim-move')
let g:move_key_modifier = 'C'
```