Add DynamicIcon as an alternative to vbars.

* DynamicIcon is an Int -> String which takes a value from 0..8
  and produces a filepath to an icon.
* API is similar to that of vbar.
* Default parser of DynamicIcon produces a function of `i` to a string
  that replaces all occurences of "%%" with `show i`
Implement DynamicIcon for Monitors supporting vbar.

* Batt
* Bright
* Cpu
* Disk
* MPD
* Mem
* MultiCpu
* Volume
* Wireless