AR-15 grip

[dalelerah](https://www.thingiverse.com/dalelerah/designs) made and tested a working model: [https://www.thingiverse.com/thing:6377480](https://www.thingiverse.com/thing:6377480)

Remix of AR-15 Grip by mr2quik

added features:
- mounting screw hole and channel
- fire selector detent hole
- slightly widened the channel that mates with the lower receiver

I haven't tested the model yet. Will update once the fitment of the holes has been fine tuned.