Add search result highlight to search result highlight display

[andybroomfield]

Fix #97

Usual cavet about using search_api as dependency applies. (If not using search_api, the search result display will be blank)

[finnlewis]

@ekes would like us to add a test for this, following the example on https://github.com/localgovdrupal/localgov_step_by_step/pull/64/files#diff-afe33a5fb745c57e43f66de12ec75bfae6c2526c9e584f3e522a73eeb73800e6

[finnlewis]

@ekes this new test look ok to you?