Make stopwords configurable

[fpompermaier]

It would be nice to allow to configure the list of stopWords. Upgrading Lunr version (like said in #28 and #30) would be great

[cigolpl]

@fpompermaier it's possible in ItemsJS 2.1.1. Example of Lunr 2.x integration -> https://github.com/itemsapi/itemsjs/blob/master/docs/lunr2-integration.md