Update 2013-12-15-common-vim.md

Some slight rewordings and added a few things.

At first, I was going to put the mnemonic of the commands in the headings, but then I realized it's more of a dictionary-style thing.

This is obviously not for a beginner, maybe an intermediate beginner who knows his way around?

Also might be interesting to have the inverse of this, i.e. "I want to do X, how do I do that?"