Make MessageValue subclassable and allow passing it as input variables

[stasm]

I'd like to suggest that format and formatToParts take a dictionary whose values are either unknown or instances of MessageValue. This allows two interesting use-cases:

1. Pre-set certain formatting options on input values.
2. Pass instances of custom types which extend MessageValue.

Then, you'd first check if an un-annotated value is an instance of MessageValue. If so, carry on. If not, resolve to MessageUnknownValue. This way, you don't have to try to resolve every expression to a number and string first.

Originally posted by @stasm in #22 (comment)

[eemeli]

Currently, the proposal allows for inputs with object values (for example, see step 7 of MessageFormat Number Functions) that may have a valueOf() method and an object property.

Is that sufficient to serve the use cases presented here?