Figure out how the pre-build step(s) should work

[gedankenexperimenter]

This should probably wait until I've got a few more plugins working, so I can test it out.

The first thing to do is to see if a header file in the sketch dir can be included from plugins (probably with weak symbol defaults defined in the plugin, if possible). I want the pre-build to only affect the sketch files, not anything in the libraries.