Specification Url for custom scalars

[phillip-kruger]

As per discussion in #167 :

Date, Time, DateTime are definitely more useful but I would recommend to specify them via specification url. The hope is that the graphql ecosystem will agree on some Date related scalars clearly specified by a spec.

Once the spec support "Custom Scalar Specification URLs" (see graphql/graphql-spec#635 and graphql/graphql-spec#649) we need to make sure that our own internal Custom scalars (Time, Date, DateTime, BigDecimal, BigInteger) support it.

We also need to include this when defining how to create a custom scalar in #94