Add search term highlight functionality in documentation

Search terms need to be highlighted in the documentation. This requires some Java scripting in our docs template, as we would like this to happen in the documentation. 

Several options are available to do this. Most involve wrapping matched terms in mark up that we can style. Here are some of the threads I've discovered:

https://github.com/olivernn/lunr.js/issues/97

https://github.com/olivernn/lunr.js/issues/97#issuecomment-182813369

This comment has a demo implementation:

https://github.com/olivernn/lunr.js/issues/25#issuecomment-270369078

Demo: https://olivernn.github.io/moonwalkers/