html command missing from --help but included when no command supplied

[drjasonharrison]

When executing

mutmut --help

the html command is not listed.

However, executing mutmut without any commands lists html as a possible command.

$ mutmut
Usage: mutmut [OPTIONS] [COMMAND] [ARGUMENT] [ARGUMENT2]
Try 'mutmut -h' for help.

Error: None is not a valid command, must be one of run, results, apply, show, junitxml, html

Also html is not described in the README.md.