Add an "explain" mode for the package search

Should be useful for tweaking the search paramaters.

Example:
http://hackage.haskell.org/packages/search?terms=process+monitor&explain
Add non-term features to the search engine

But not yet used for any such features
Include more haddock markup in package search

In the package description, index search terms appearing as
haddock-markup identifiers or single-token code markup. For example
allow 'ByteString' and @text@ markup as search index terms (after
stripping off the markup obviously).
Add download counts to the search ranking

No parameter tuning yet.
Add download count to search explain/tweak page

So can see the values and can adjust the weighting.