Support custom Scalar URLS

In terms of using custom scalars by URL, we have support now for `specifiedBy` directive!

Now users can host common scalar definitions, etc.

Any tickets you see that ask about using some custom, non-spec scalar type, whether closed or open tickets, should all be directed to this ticket.

Here is the spec:
http://spec.graphql.org/draft/#sec-Scalars.Custom-Scalars

so this works as long we use `specifiedBy` (SDL) or `specifiedByURL` in the introspection query (how should we expose adding this field in graphiql? or add it every time? is it there already).

If this is in the schema, then the inline variable validation should work automatically based on our usage of graphql validate(). However, in both `codemirror-monaco` and `monaco-graphql` we have more specific logic for validation of variable scalar values that will need to be tweaked for both plugins.