Add a few key mappings to vimrc

<leader>l toggles whitespace highlight and <leader>A does an ack for
the word under the cursor
Squashed '.vim/bundle/ack/' content from commit f183a34

git-subtree-dir: .vim/bundle/ack
git-subtree-split: f183a345a0c10caed7684d07dabae33e007c7590
Merge commit 'c106419b947ee34cf94ff95c933e29207b88d3ba' as '.vim/bundle/ack'