vimrc: use Ctrl + arrow keys to move among splits
vimrc: use Ctrl+o for single normal-mode commands

If you're in insert mode, and you need to enter normal mode, execute one
command and then go back to insert mode; then it is better to do it via
'Ctrl + o' instead of <ESC> and then i/a.

Refer:
http://stackoverflow.com/a/3985410/793930