Add module group feature

[njoyard]

This PR adds a module group feature. A module group contains multiple modules and renders them orthogonal to the bar direction (ie stacked vertically for a horizontal bar and vice versa).

My use case for it is that I want a vertical bar but I want some modules to share the same row on the bar. This may sound like a waste of screen space but I'm using waybar as an overlay that is most often hidden, and show on demand. A screenshot of my setup follows, volume/brightness bars are implemented each with two modules (one for the icon, one for the bar) placed in a group. Progress bars use CSS linear gradients as their background.

The way a group is configured is as follows:

{
  "modules_left": [
    "module_a",
    "group/mygroup",
    "module_b"
  ],

  "group/mygroup": {
    "modules": [
      "module_c",
      "module_d"
    ]
  }
}

I chose to list modules in a "modules" key to leave space for future additional options for groups (eg. alignment and so on - which would probably be useful for right/bottom-positioned bars). The implementation does not prevent nested groups, but that's currently not very useful as a group direction is orthogonal to the bar, not to its parent.

Please note that I have no significant experience with c++ whatsoever, so my code may leave a lot to be desired. I've been using it as a daily driver for quite some time though so I'm hoping any remaining issues are mostly architecture/style/good practices related.

Let me know what you think.

[Alexays]

Nice addition!
Can you document this in man? :)

[njoyard]

Added some documentation in the main manpage, and rebased on master.