Allow user to specify alternate config file with --config flag.

[bit101]

Use case:

Sometimes I want to use my laptop keyboard to control M8. Other times I want to use an external keypad I've set up to mimic a hardware M8. This allows me to have two different configs and choose which one to use when I run m8c.

./m8c - runs with the default config, config.ini

./m8c --config config-keypad.ini - runs the alternate config specified.

Notes:

• The alternate confg(s) are expected to be in the same config path as the default one.
• If a user specifies a config that does not exist, it will be created with a default config, which they can then edit.

[laamaa]

Looks useful, thanks! Would you mind editing the README.md too to document this flag?

[bit101]

Sure! I also noticed a TODO comment in the code for something similar to this. I can remove that too.

[bit101]

Updated readme and removed comment.