fn update context allows arbitrary keys

[jlbutler]

Arbitrary keys may be added to a context via 'fn update context'. This may or may not be as intended, but it adds cruft where a use case is hard to discern. Possibly the call should check for the list of used / valid keys.

[rikgibson]

The namespace of available/valid keys really depends on the full range of context providers (admittedly only two at the moment) and then where do you go if you start validating keys to fn update context - do we start validating the context files? I get the point about cruft but invalid values just get ignored but I think we have bigger fish to fry.