Add force mode to TagCommand.

There are actually two force modes, one to force adding a tag and
one force mode that omits user interaction. The first kind is done
with -a, the second one with -f.

The meaning of -f changes in this commit, but now it's consistent
with the other subcommands.
Add classifiers and a long description to setup.py.
Print the getopt error.
Adapt negation test cases to work properly with getopt.

When using negation, the "--" separator should be used, so getopt
doesn't stumble over -word constructs.

Also add a test case that checks functionality of a negation with
multiple words.