Highlighting can return excerpt with no highlights

You can configure the highlighting api to return an excerpt of a field
even if there wasn't a match on the field.

The FVH makes excerpts from the beginning of the string to the first
boundary character after the requested length or the boundary_max_scan,
whichever comes first.  The Plain highlighter makes excerpts from the
beginning of the string to the end of the last token before the requested
length.

Closes #1171