Support config overrides in context configuration

#### To request a feature...

* Clearly describe the feature.

Context configuration in ```.taskrc``` doesn't respect ```rc.XXXX``` overrides.

* Clearly state the use case. We are only interested in use cases, do not waste time with implementation details or suggested syntax.

Contexts could benefit from being able to support custom sorting on reports for that particular context.

It would be great if this worked:
```context.biz=project:Biz rc.report.next.sort:project+/,urgency-```