Add DynamicString as an alternative to vbars.

* DynamicString is an Int -> String which takes a value from 0..8
  and produces a string with this value embedded. Usefull for replacing
  vbars with icons: an icon per one of 9 states.
* API is similar to that of vbar.
* Default parser of DynamicString produces a function of `i` to a string
  that replaces all occurences of "%%" with `show i`
Implement DynamicString for Monitors supporting vbar.

* Batt
* Bright
* Cpu
* Disk
* MPD
* Mem
* MultiCpu
* Net
* Volume
* Wireless
Add documentation