add subtotal plugin

[timelyportfolio]

I will volunteer to do this, but wanted to file an issue to track progress. Add subtotal plugin. @smartinsightsfromdata, any objections?

[nicolaskruchten]

This should be fixed in version 0.3.0 of this library! https://github.com/smartinsightsfromdata/rpivotTable/releases/tag/v0.3.0