Add additional events for gcode execution

[Faultier523]

It would be cool to add a possibility for a script when a filament change is required, for example to automatically move to a position and eject the filament.

[smartin015]

Agreed, that would be cool. I think I'll expose this after I've refactored script-running to be a bit more general, though... currently each script/event requires code and tests to be added in several places before they work. That's OK when there's only a couple scripts, but it seems like "on queue event, run script" is a recurring feature request :)

Edit: Per #124 we have another potential event-driven gcode script. I'll start a record here:

• Run gcode on filament change required
• Run gcode before starting print

[smartin015]

With submission of #149, these events will be available to run scripts on in v2.3.0 - release ETA is probably some time in December.