Add helper to highlight and excerpt by regex

Backport of https://github.com/rails/rails/pull/11793/

Contains integration tests to check that it works
as expected with ActsAsXapian.
Use regex based highlighting
Add spec for highlight_and_excerpt
Handle unhelpful stemming

Stemming returns 'bore' as the word to highlight which can't be matched
in the original phrase.

Also removes duplicates from the results