Add json.Number support to UnmarshalString

[Desuuuu]

UnmarshalString currently supports most basic types (string, int, float64, ...), but not json.Number, which is used when decoding float values from JSON.

This PR:

• Adds json.Number support to UnmarshalString
• Implements graphql.UnmarshalString tests which were missing
• Changes the behaviour of graphql.UnmarshalString with float64 inputs to output a string with the smallest number of digits necessary (graphql.UnmarshalString(float64(123)) will output "123" instead of "123.000000")

The last change while not strictly necessary seemed to make more sense when I was writing the tests. I made it in a separate commit so it can be easily discarded.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

Coverage increased (+0.2%) to 75.641% when pulling b14a945 on Desuuuu:unmarshal-string into daa4407 on 99designs:master.

[StevenACoffman]

Thanks!