Customizable name keychain

This is a customizable keychain generator. The keychain is made out of a word. If the word has a letter with a nice hole, you can omit the ring for the keyring. This uses (and bundles) my [Hershey font](https://www.thingiverse.com/thing:2404566) OpenSCAD library.

Note the licensing on the Hershey library in the source code, which isn't fully compatible with Creative Commons -- but the restrictions are very, very slight. And it's not clear that font coordinates are copyrightable (in the US at least; but I am not a lawyer).

**UPDATES:**
**August 16, 2019:** optimizations in Hershey library; different ring cross-sections
**August 10, 2019:** uses latest Hershey font library: nicer font names, Latin-1 accented characters supported
**August 23, 2017:** added rounded top option
**August 19, 2017:** smart ring and letter spacing (turn off smart overlap for legacy letter spacing); works much better with combinations like "Ta"
**August 15, 2017:** fixed "NAME" bug (again!)
**August