Customizable Filament Gland v2.1

## v2.1 update 2020-05-10:

* added M10 threaded inner plate variant, for use with PC4-M10 fittings; updated instructions and hardware list
* added a couple of stls featuring the M10 inner variant

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Storage box filament gland with two holes. This version has a customizable number of filament openings. This version also fixes two problems I had with the earlier version:

1. It's now much easier and faster to print, requiring no supports whatsoever
2. Filament moves through it much m