Filament Runout Sensor for Titan

**Update 2018-03-05:** I've added a new thicker shim piece (`Filament_Runout_Sensor_piece_thick.stl`). It's meant to be used with a limit switch that has the metallic lever removed (squeeze and twist it off with pliers). It's *highly recommended* over the original piece.
My limit switch started to wear out and became slightly unpredictable, but removing the level and using the thicker shim is excellent! Shame I didn't think of it in the first pace.

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A [filament runout sensor](http://m