Add option to pseudonymize the common name

[phihos]

I finally got around to implement the feature. The changes were bigger than expected. Sorry about that in advance.

The command module now has end-to-end tests. For that to work in the same process I had to make some changes to the command module itself. But they should be functionally equivalent. Since the changes only work on newer golang versions I had to update the minimum version.

The pseudonymization feature itself is implemented as decorator for parser.ParseFile. If applied it intercepts the Status struct and pseudonymizes each CommonName field in the ClientList.
As promised everything is fully tested. I did not touch any preexisting test so you can be confident that the previous behavior has not changed.

Closes #19