Added interfaces/email/interactive/README and be-handle-mail options.

Added interfaces/email/interactive/README and be-handle-mail options.

The README should give enough info to install and use the interface.

While I was writing it, I thought that be-handle-mail could use the
--be-dir, --tag-base, and --test options.  generate_global_tags()
helps implement the --tag-base option.

I set up a unittest framework since checking is currently a
pipe-in-emails-by-hand sort of arrangement, which can be slow ;).
Currently only generate_global_tags() is tested.

I also restored "show" to ALLOWED_COMMANDS, since it seems to have
wandered off ;).