Ideas for search bar improvments

[tristanlatr]

• give better ranking when an object matches multiple clauses
• astutils. should list all members of the pydoctor.astutils module (translated to astutils.* *.astutils.* in lunr syntax)
• t.w.t.tag should be translated to t*.w*.t*.tag* *.t*.w*.t*.tag* in lunr syntax
• Change the default logic operator in between clauses: Currently all clauses are ORed together, so the number of results will increase every time users enter a new clause. Which is not a great user experience IMO. We could ANd the clauses by default and add a button to change the search logic.