Also updated CONTRIBUTORS.md, added a "none" option for
`core.logoutput`, and added type annotations to the Skylark struct

This closes #47

Compile time still increased to 50s (from 15s), but at the moment I
can just about live with that.

Unfortunately, including Makie as a dependency increases the compile
time to ~5 minutes. This is unacceptable and I need to find a fix for
it, if possible.