Add Plug for ctrl mappings

17f084a53082be7b261be5a7cf97c95c36622bfe prevented me from using the Ctrl-p/n maps that I like, so I made them Plug mappings that can be defined in a user's ~/.vim/after/ftplugin/gitv.vim

Also updated the modeline to match your whitespace settings and fixed a few indentation lines that didn't match the rest of the file. (I left the fugitive lines alone since they're copied straight from fugitive.)