Configurable color/monochrome mode from CLI and config file

This is a job for a junior browsh developer and should be possible to solve for anyone who has the development environment setup.

Currently the monochrome mode can only be activated by pressing ALT+m.

The default should be configurable in the config.

And it should be possible to select the color mode from the command line.

Use cases:

• For testing purposes I was using ansi-term inside emacs today and unfortunately the key combination ALT+m isn't forwarded correctly to browsh.
• Prefering monochrome mode to save bandwidth