Makefile: add "make tag" command

Makefile: add "make tag" command

Add a new "make tag" command. This automates some common operations:

1) Automatically determine the next Git tag version number to create.
   For example:
   "3.2.0beta1 -> "3.2.0beta2"
   "3.2.0rc1 -> "3.2.0rc2"
   "3.2.0" -> "3.2.1"

2) Create the Git tag, and print instructions for the user to push it to
   GitHub.

3) Sanity check that HEAD is a stable-* branch or master (bail on
   everything else).

4) Sanity check that HEAD is not already tagged.

Note, we will still need to tag manually once each time we change the
format, for example when moving from tagging "betas" to tagging "rcs",
or "rcs" to "stable point releases".

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
Co-authored-by: Guillaume Abrioux <gabrioux@redhat.com>