New option: "Prefix Anchor".

This adds an option called "Prefix Anchor" which will require the completion items to match the first "x" characters you type ("x" being the integer you set). Defaults to `0` (or `off` by default).

I added an explanation of the option in the README.

Also, as I was testing, I noticed that you cannot set defaults for Xcode using `defaults write com.apple.dt.xcode`.  Instead, you must use `defaults write com.apple.dt.Xcode` (notice the uppercase "X"). I modified the README to reflect this.  I suspect some users find the options don't work like they should because they're not setting the defaults correctly.