Customizable folding phone/tablet stand

This is a remix of the excellent Universal phone/tablet stand with adjustable angle by ClassicGOD, with the following enhancements:

- Most parameters are now customizable (via the attached OpenSCAD file),
- Added a stopper (customizable height) since my phone kept sliding off from the original,
- The phone holding area is also customizable to accommodate thicker or thinner phone/cases
- Added an optional hole to the phone holding area to reduce rocking with phone/cases with round bottoms (