convert: Add separate "replace" configuration

[diomekes]

This should be part of #232, which is closed, but along with paths: in the convert plugin I think a separate replace: would be nice as well.

[sampsyo]

That implementation (#232) added a fair bit of complexity to thread through different replacement configurations. So if we do this too, it might be nice to go whole-hog and use a more general approach. Something like generically duplicating a subset of the top-level configuration options in the convert: section. (Not sure how this would work, but we might be able to find a more principled way to do it.)

[diomekes]

That sounds like the best way to do it, because I just realized that I also need to change pathfields: each time I do a convert.