Add switch unused feeder(s) off

Having the non-active feeder motors powered on all the time is not
necessary. A feature to deactivate the unused feeder motors has been
implemented. The feature is enabled on default but can be switched off
in the configuration.
renaming the disable inactive extruder feature
Merge pull request #947 from Dim3nsioneer/Marlin_v1

Add feature 'switch unused feeder(s) off'