--help should document options to --encoding

I just ran into an issue with csvjoin 1.0.5 where a file was failing to parse because

UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc1 in position 377: invalid start byte

It didn't like Á.

In the end the solution was --encoding iso-8859-1, but it was really frustrating that the help said there was an encoding flag and nothing about how to use it.