Extract WP_CLI_VERSION to a separate file

*This is a prerequisite for writing Behat tests for #1535*

- Implements #680
- It is now possible to set the version at build time (required for #1535): `--version=x.y.z`
- Keywords for incrementing the version number by one: `--version=[patch|minor|major]`
- When no arguments are passed, the old behaviour is preserved
- `utils/update-phar` takes an optional argument: `update-phar [same|patch|minor|major|x.y.z]`
It would be good to break this logic into a standalone function, and include unit tests with it.