add support for only (re)generating module file

This adds support for only (re)generating a module file. The sanity check is still run, to make sure that the software installation was already taken care of, unless `--force` is used, then you can also generate for non-existing installations (which is great when experimenting with a particular module naming scheme).

One thing which is not possible (yet) with this is to generate module files for existing installations using a different module naming scheme, since the installation prefix always matches the module name being used.

This could probably be fixed as well, but needs extra logic and probably and extra command line parameter, to specify using which naming scheme an existing installation was installed (so the software installation prefix can be found).
does it make sense to generalise this to a step `whitelist` (the skip option being the step blacklist equivalent)?