When revng cli was invoked with a bare namespace, e.g. `revng ptml`
it would produce an ugly exception.
Fix this behavior by instead rerunning the command with `--help`
appended.
This commit introduces some miscellaneous fixes to revng.cli:
* Remove useless subclasses in pipeline_tools.py;
* Improve type annotations in _commands/ptml and fix a help string;
* Add some extra type annotations for `Command`;
* Remove useless namespaces in `commands_registry` and add help text
for the ones in use;
This Oython tool will be used to easily work with PTML.
It currently features two subcommands:
* `cat` prints a PTML to the terminal, with color formatting if
possible;
* `strip` will strip a file of the PTML markup;
Giacomo Vercesi