feature: _Font.strike_through

[amisa]

Added _Font.strikethrough

[scanny]

Hi @amisa, thanks for this.

I can't commit it as is because there are several pieces missing, but I'll keep it on file here for when we get to this feature.

Just so you know, in order to be committed, a feature needs acceptance tests and unit tests, and in this case I believe will need a new enumeration.

Still, it's handy to have an example implementation when we get to it, so we're grateful for your having sent it along :)