Automatic version check

[edjeavons]

We should start versioning this repo, then each time a command is run (especially create-site) the package could check if it's running on the latest version and fail gracefully if not.

It should exit with a helpful message such as:

Your copy of the eighteen73/wp-cli-tools package is out of date.
Please run "wp package update" then try again.

[edjeavons]

I've added some code that I think will work but it's hard to test locally without tagging versions of this remote repo.

I'll leave this issue open and return to it if it looks like there are problems in practice later.