previously we just used a Go builtin map
this was not safe for concurrent read/write and upon upgrading
to Go 1.6 we were notified of the problem
fixes#349
this started initially to relocate highlighting into
a self contained package, which would then also use
the registry
however, it turned into a much larger refactor in
order to avoid cyclic imports
now facets, searchers, scorers and collectors
are also broken out into subpackages of search
Marty Schoch