User defined custom types

[vektah]

Fixes #1

Ended up being way bigger than I expected.

Allows the user to:

• define custom types using the standard go style Marshaler / Unmarshaler interfaces.
• define custom marshalers on builtin types, see the example for details.
• redefine any of the builtin scalar types

It also removes the runtime dependency on mapstructure by building custom unmarshalers for all input arguments, resulting in faster execution and more type safety.