Feature Request: Allow initial non-extruding moves into excluded region

[bradcfisher]

Follow up to #52

When entering an excluded region, (almost) all commands are prevented from being sent to the printer. This leaves the extruder potentially sitting over a previously printed part, which can be damaged due to the heat and/or oozing of filament.

The suggested enhancement is to allow any initial non-extruding moves (including moves with retraction) to be sent to the printer after entering an excluded region. This behavior should cease as soon as the first extruding command is processed in the region.