Improve single backticks to distinguish code, arguments, keywords

[fingolfin]

Motivated by the discussion on PR #190, I am hereby splitting the request for single backtick support from issue #184 into this new issue with more details:

It would be great if our single backticks code could distinguish between keywords, arguments and the rest, and then depending on that insert <K>bla</K>, <A>bla</A>, or <C>bla</C> -- right now, it always inserts <C>bla</C>.

[fingolfin]

Note that one can use ALL_KEYWORDS() to detect keywords. The hardest bit is to recognize arguments. One problem with that is that we may not know them yet at the time we process the backtick.