yabar: add module

To make the configuration of `yabar` more pleasant and easier to
validate, a NixOS module will be quite helpful.

An example config could look like this:

```
{
  programs.yabar = {
    enable = true;
    bars.top.indicators.exec = "YA_DATE";
  };
}
```

The module adds a user-controlled systemd service which runs `yabar` after
starting up X.