Make it so `fillin` can be filled in

[Alex-Jordan]

It might be good to allow a "fillin" to be printed filled in. For example in a statement: "bowling uses _____ pins" but in a solution you have the same thing (including the underline/box/shade) but with "10" filled in.

Potential for abuse as a way to "underline" some text.

[rbeezer]

I'm faking it in a solution with a 1-character fillin just before and just after. Written automatically as the solution is generated by the pre-processor.

I realize now that this device will be subpar with shade and a bit screwy with box. Runestone auto-generated static bits are a work-in-progress. ;-)