Module themes (only!) can now contain state-specific theme information -
for example, the "battery" module has different states for charging and
discharging, and those can have different prefix and postfix
configurations to indicate what is going on.
Add a plugin that displays the remaining battery power in %. This also
introduces the concept of arguments that can be passed to a module
during startup by delimiting the module name with ":", for example:
-m battery:BAT1 to query the BAT1 device.
Note that this works to an arbitray length, i.e. if a module accepts 3
parameters: -m <modulename>:<A>:<B>:<C>
The module gets the arguments as list.
