Improvements to the post processor

[mariusbarth]

Is your feature request related to a problem? Please describe.
I think it is tedious to always write Figure\ \@ref(fig:chunk-name), i.e. I forget to write the non-breaking space, frequently.

Describe the solution you'd like
maybe we should explore the possibility to run a gsub() in the post processor.

Describe alternatives you've considered
maybe this is something that would be interesting for the bookdown authors and should be considered there.

Additional context
Add any other context or screenshots about the feature request here.