Add sensible defaults to vimrc.

Used these examples as references:
  https://github.com/skwp/dotfiles/blob/master/vimrc
  https://github.com/tpope/vim-sensible/blob/master/plugin/sensible.vim

For documentation on the available options, see:
  http://vimdoc.sourceforge.net/htmldoc/options.html
Add initial keymap.