Multiple mappings in one mapping file

[edemaine]

Especially when we have combining mappings (#83), may want to return multiple things from one mapping file.

Most naturally, we could export an array of things, as long as each one is distinguished as to its type (instead of or as alternative to named exports).

Update: Exporting an array of things now has a specific meaning, which is essentially combining mappings of #83: apply everything in the array and stack them on top of each other. This is great for combining mappings, but we still lack a convenient way to say "apply this mapping, and if it doesn't find anything, apply this mapping".

Alternatively, we could use callbacks like svgtiler.mapping(...).