Update HISTORY section of docs

To include info about commit 2f6e86520c5e.
Add g:CommandT{Ignore,Smart}Case

Originally submitted as a pull request by Jacek Wysocki here:

  https://github.com/wincent/Command-T/pull/12

I rewrote it to apply on the current HEAD of the "next" branch, added
a smartcase option, and renamed (and inverted the sense of)
`g:CommandTCaseSensitive` to `g:CommandTIgnoreCase` to more closely
track Vim's `'ignorecase'` and `'smartcase'` settings.