feat: adds option to hide clean flag

[joshmedeski]

Purpose

Adds the option for a user to hide the clean flag. I, personally, would rather not see anything as an indicator that everything is clean.

Approach

Add a hide_clean option that defaults to false and when set to true will not render the clean flag.

[arl]

Thanks @joshmedeski
Can you add some test please?
No need to be fancy, just a single test case showing off the feature.
Many examples are
https://github.com/arl/gitmux/blob/main/tmux/formater_test.go

[arl]

Thanks a lot for doing that.

Nothing to add on the code side.

It would be nice to document the new flag.

in the default documentation:

• add the option in the default documentation file printed by gitmux --printcfg. With a comment

In the README.md:

• update the default documentation. Without comment.
• update additional options table

Thank you

[joshmedeski]

All done, thanks :)