Add --version option

This also adds the necessary infrastructure to the makefile to track the
version number and ensure that the script is rebuilt when it changes.

The initial version has been set to 0.1.

Signed-off-by: John Keeping <john@keeping.me.uk>
Merge branch 'version'

Add some infrastructure for a version number and a --version option to
display it, in preparation for actually tagging a release.

* version:
  Add --version option