Snap Assembly Cable Passthrough Grommet

## Customizable Snap Assembly Cable Passthrough Grommet

### Update 12/22/20
Updated customizer (openscad) script to version 5.4, with the following changes:

* Grommet diameter value now refers to the outside diameter, instead of the inner bore diameter as in previous versions. Now the diameter value matches the size hole it will fit. (ex: a 20mm grommet fits into a 20mm hole)
* Ring on top of snap is slightly looser than in previous version, making it easier to apply.
* Ring on top of plug is smaller than in previous version for the same reason.

### Description
This is a remix of a tarp grommet. I made two main changes to this remix, in order to make it suitable for passing wires through it and make it easier to place the snap on and off:

* Internal plug: There is a plug that fits into the internal bore of the grommet.
* Strain relief slits: There are 5 slits in the wall of the grommet, to make it easier to fit the grommet through holes in rigid materials like metal or