allow separate ladspa-plugins for sink/source processing

[umlaeute]

sometimes it's too complicated to have a single-ladspa plugin that provides both source&sink processing. instead it would be good to be able to specify two separate plugins (for source resp sink), e.g.

sink {
  library "echo.so"
  module "echo"
}
source.library "dereverb.so"
source.module "drev2"

in this case, it is save to omit the source.inchannels (and friends) configuration, as this info can be derived from the ladspa-plugin)