add an -o auto to build a variable or static TTF depending on number of masters

[anthrotype]

currently with -o variable, we end up needlessly building empty or no-op variable tables when the source only contains 1 master.
Adding a new -o auto would align fontmake output options with fontc, which would aid comparing the two programmatically.

see googlefonts/fontc#971 (comment)

While we could change -o variable to instead build a static font when len(masters) == 1, the two variable vs static code paths are distinct enough in the fontmake pipeline to warrant a separate -o parameter. It's not the same thing to build a single master for a not-so-variable font or build it as a static font (e.g. think removing overlaps).