Autolink code in backticks

[hadley]

Assuming that it parses; and probably only to functions in current package (at least to start with)

[hadley]

Can now use downlit.

[hadley]

Can't really use downlit here because it generates URLs not code links.

[hadley]

No longer think this is a good idea. Markdown linking is already very easy and often you only want to link the first usage of a function.