add check command to the confusable_homoglyphs CLI

[pabs3]

It would be useful to be able to check files for confusable homoglyphs on the command-line. While it is possible to write a python3 -c command, it would be much more ergonomic for the confusable_homoglyphs wrapper to have a check command that would check filenames in the arguments for confusable homoglyphs. Perhaps an argument of - could cause the command to check stdin, or maybe it should check stdin when there are no filename arguments.