added highlight to element on hover by decreasing the opacity of the bac...

...kground elements using a global higlight function

@jebeck can you look at it, i think its ready but i'd like to hear what you think of the code and the resulting interaction.
I don't think passing the selection as a string is the best option here. The problem is that we could at some point have multiple tideline instances on the same page, and so we need to avoid global selections that chain off `d3.select/selectAll`. I think it'd be better to set highlight up with the actual selection...and I think it'd be best to move the highlight setup into `pool.js` as a getter/setter so that each plot module can get the appropriate highlight function with `pool.highlight()`. Also, the getter/setter should use things like `pool.id()` instead of `#poolBolus`. Same for the classes, but that will have to be added in `pool.addPlotType` (add a class attribute so that boluses have a type of `bolus` and class of `d3-bolus-group`). Then you should just be able to iterate through the (diabetes-related) plotTypes (we may also need to create a diabetesData: true/false property in each plotType) for each pool to generate the `backgroundSelection` for highlight. Let me know if this makes sense - if not, let's chat!