Support dmenu-rs and support all base flags for dmenu extension.

[thebashpotato]

Please describe the change / addition you'd like to see made

• Is this completely new functionality, or an enhancement to existing functionality? Enhancement to existing functionality.
• What kind of feature is it that you are requesting?
  • A custom extension

Problem

I personally use the dmenu-rs port of dmenu. The plugin system is better than old-school patching. The problem is the flags are slightly different.
For example the following flags [ -nb, -nf, -sb, and -fn] use a single dash in Suckless dmenu,
but use a double dash in dmenu-rs, which will cause it to not to launch. Also the ability to use a custom font
in dmenu with the fn flag would be nice as to match the bar, and the -b flag would be nice to allow users to place dmenu on the bottom of their screen.

Proposal

Instead of implementing a new DMenuConfigRS extension, modify the current DMenuConfig struct
to account for these changes. I have already implemented all of this functionality and also changed the DMenu API slightly to be a bit more readable and make a clear distinction between launching dmenu via dmenu_run, or launching a menu based dmenu.

Here are the changes in my penrose setup includes some tests. And here they are used in actions. I could merge this into upstream if it sounds like a good idea @sminez