Added option g:markology_disable_mappings

This is a really nice little plugin. I've been using it so that I can get better at using marks. However, I find it annoying when plugins create default keybindings. I rarely use the defaults, and don't want to go to the trouble of exhaustively remapping everything just so I can get my precious ```<leader>m``` back.

This patch lets me disable the defaults by doing a simple 
```viml
let g:markology_disable_mappings = 1
```