Figure out cross-references

[hadley]

From @jjallaire

(1) Inherit from bookdown::html_document2()

(2) Call the helper function I asked Yihui to put in so I could support cross-references in distill

(3) Look at the code of that function and just do it yourself (that might be the best bet although I haven't actually looked at the code recently so don't recall how trivial or non-trivial it is)