makefile: integrate xyz

This cannot be merged as is. The problem is the automatic CHANGELOG generation, which requires knowledge of the new version number. Perhaps [xyz][1] should accept a path to an arbitrary script to be run immediately after the confirmation prompt. This script could be passed the new version number as $1.


[1]: https://github.com/davidchambers/xyz

Updated to use the same shorthand as davidchambers/xyz#15.