Add configurable option for 'total' lines on time series graphs

Pivotal: https://www.pivotaltracker.com/s/projects/911874/stories/63885814

Add a new configurable option in Stagecraft stubs called "show-total-lines". If it is set, create a "total" series when parsing data. 

Some assumptions: 

* The parse method assumes that data objects have a `_start_at` property. 
* The Stagecraft stubs assume that you will hard-code a "total" object in "series", as well as setting the `show-total-lines` option. I think that's OK since we're already hard-coding the names of the series in these stubs.