Support booleans and numbers for query values #141

It'd be nice if non-string values in a query were automatically stringified:

list_fish:
  method: GET
  url: "{{host}}/fishes"
  query:
    big: true

Right now, this gives an error because true is a boolean and it expects a string. It'd be nice if booleans, ints and floats all deserialized into their stringified version.

I think the best way to do this would be to implement Deserialize manually for Template. I would put the implementation in cereal.rs to keep all the serde stuff in one place. This has the added effect of automatically supporting bools/ints/floats for any other templated value (body, profile value, etc.) which is probably good but needs a bit more critical thought to be sure.