Blessed tags & similar packages

[Chris00]

As asked in #41 (comment), I open a new issue about tags.

One idea is to have a set of "blessed tags" to categorize packages (otherwise, as seen on ocamlforge, people will use slightly different tags to mean the same thing).

Another thing is to use tags (and possibly the description) to suggest similar packages. This is more difficult as it requires some code to define what "similar" means.