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 highlight on the field.  This must be implemented
in each highlighter individually.

The FVH implementation seems to do a good job of building the excerpt.
The Plain implementation I'm less happy with.

Closes #1171
Minor parsing tweak.
Rename highlight no_results to no_matches.

Closed #1171
Documentation for highlight no_matches.

Closes #1171