This is a change in search result behavior in that location
information is no longer provided by default with search results.
Although this looks like a wide-ranging change, it's mostly a
mechanical replacement of the explain bool flag with a new
search.SearcherOptions struct, which holds both the Explain bool flag
and the IncludeTermVectors bool flag.
Boostable, Fieldable, Validatable broken out into separate
interfaces. This allows them to be discoverable when
needed, but ignorable otherwise. The top-level bleve package
only every cares about Validatable and even that is optional.
Also, this change goes further to make the structure names
more reasonable, for cases where you're directly interacting
with the structures.
