cleanup: Remove 'status_bar_' config prefixes

[cmacrae]

Hey @somdoron 👋 Thanks for statusbar! I'd been looking to do the same thing, so I was delighted when someone pointed me to your repo.

I figured as this project is now simply a status bar on its own, the
status_bar_ prefix on all config options seemed superfluous :)

So, hopefully this simple cleanup is a welcomed change

[cmacrae]

Noticed the manpage didn't include the clock_format option, so figured I'd make man and include it here 🙃

I can squash my commits if you'd prefer

[somdoron]

Thanks!

[somdoron]

For the three people that already use this, we might want to have a backwards compatiblity?
We can even not document it, just accept anything that prefixed with status_bar_