Expandable snap-fit USB holder

### Design principles
- This USB holder can be joined with another unit by snap-fit mechanism(https://youtu.be/GeIpu0_ty3o).
- There are hundreds of USB holder models available, but I didn't want to make 10 slots while I needed only 2 or 3.
- I hate support structures so I avoided any overhang larger than 45 degrees. You don't have to print any support structures.

### How to Use
- To connect the units, just push them until it clicks.
- You can disconnect units by one of the following methods:
- Pinch the side walls(the roots of snap-fit hooks) firmly
- Twist them slightly while pulling one from the other
- Push the hooks directly through the hole on the side walls. You may use 1.75mm filament pieces.

### Description of STL files
- snap_fit-default: If you don't want to be bothered, just print this as many as you want/need.
- snap_fit-no_groove: Place this at the end of your USB holder array if you find the grooves for the snap-fit is ugly. This model doesn't have the groo