Better multi-monitor support

[juster-0]

This changes add ability to set specific monitor for the bar. I've only added xrandr lib, that probably already installed in almost all systems with X11.
Now you can set specific monitor to any bar in a config with option monitor. If monitor option is defined then options x, y and length will be relative to the specific monitor, otherwise will be relative to the combined screen surface.
Option monitor accepts output names from xrandr command.

[MuhammedZakir]

IMO, instead of auto, some other name would be better to reflect "combined screen surface" meaning. It's alright even if it's a combined keyword.

[juster-0]

That's a good point, I'll change it to combined, If the pull request will be approved I want to add ability to show the bar relative to cursor/active window, I think auto will be better for these cases.

[florentc]

Hello, a huge thank you to you for this long-awaited feature!! This is a great improvement.

I can't test it out until next week so I will postpone review and merge until then.

[juster-0]

Ok. I'm waiting for your review.

[juster-0]

Hello. Sorry, any updates?

[juster-0]

Hello. I currently actively work on my fork of the repo, so I want to free my master branch. I’ll create the same pull request with the same commits, but another branch, If you want you can merge it. Sorry for that and thanks for understanding.