Add -m command to cipheydocs

[bee-san]

also, may I suggest that the walkthrough doesn't suggest manually editing ciphey's source, and instead recommends the -m flag
the -m flag is rather like the python3 -m flag
it loads in the python module at the specified path
it allows people to write extensions without having to faff around with the source code
if they want to add it to the source, then they should edit the source and fire off a pull request
if they just want that locally, they can use the config file

module:
- /path/to/python/script.py
- /path/to/python/module/dir/

that config there has the two possible methods: module dir or script file
like with python3 -m
you can run python3 script
or python3 dir```

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