…duration selector

- Adds support for named arguments.
- Adds support for returning promises, et al, from the duration selector.

NOTES:

* There's a problem with type inferrence from the options `duration` selector that is noted in a `TODO` in the code here.
* The tests for `groupBy` appear to be EXTREMELY old and outdated, and I was unable to updated them easily to use run mode. We may have to rewrite them all at some point to use better techniques. The issue seems to be a rudementary means of testing the inner observables that is incompatible with run mode.
* Docs still need updated
* Older paths still need to be deprecated
* dtslint tests need to be added