feat(Renderable) Add a new Value variant: Renderable

[johnhurt]

This is a method of representing a value only though its Display or Debug implementation.

There is some discussion in #43 about making 2 different variants for Display and Debug, but that presented a problem because Valuable needs to implement Debug, but & dyn Display doesn’t. Adding a new trait like Displayable to wrap & dyn Display seems a little wasteful 😜

This method wraps both would-be variants Display and Debug in an enum Renderable and adds a new variant of the same name. The next step is to add #[display] and #[debug] derive annotations to allow struct fields to be included as their Display or Debug implementations instead of actual values (as discussed in #131)

[benluelo]

is there any progress here? this would be incredibly helpful for custom domain specific types