Add "unit" option for blocks

[domsson]

A block can have a label, which is simply text that will be displayed before the actual output of the block. One example would be "Vol." for a volume block. It would be nice to also have the option to have a suffix for every block (currently we only have prefix and suffix for all blocks via the succade config file). This would allow to define units for blocks that output things like volume (%), temperature (°C) or similar data, while not requiring the block scripts/binaries to include them in their output.

As I currently only see this being used for units, we might as well call the option "unit". This way, we have prefix and suffix for all blocks, plus label and unit on a per-block basis.