Remove stopword filter from Lunr index generation in middleman-search.

[lewisnyman]

After some testing with registers I found that #38 was only half a fix.
alphagov/registers-tech-docs#109. As the lunr index is pre-generated by middleman-search, we also need to remove the stopwordfilter there to return useful results for get.

I've added this config option in the alphagov fork. See: alphagov/middleman-search#1

[MatMoore]

How come in #38 you replace the stopword filter, but here it's removed entirely? Shouldn't it be the same when indexing and searching?

[lewisnyman]

We could do that here, and it would save us a few bytes when loading the index, but the effect is the same. I didn't want to add a list of stop words to maintain in two different file.

The other option would be to move the list of stop words into project config and then insert it into the JS file by processing it with Middleman. That's probably the best solution long-term, but it doesn't feel like something we have time for before this project ends.