Add support for "ZZ" command.

[jvz]

ZZ is an alias for ":x" (short for ":xit" or ":exit"), which itself is like ":wq" except it only writes if the file has been modified.

I can't be the only one who prefers using ZZ over :wq. If it's not too hard to contribute, I may add a PR for this.

[cldwalker]

Sorry for the delay. Happy to take a pull for this. You'll just need to add an entry in vim.behaviors