More abstract serializer and deserializer for encode() and decode()

[shah-dhwanil]

Is your feature request related to a problem? Please describe.
Currently pyseto implicitly assumes the function name of the serializer.
I am using ormsgpack for my API & I so decided to use ormsgpack but it has different alias for dumps and loads so the serializer parameter becomes useless .It would be better if could accept a function which serializes thus we can use any serialization method

[dajiaji]

@Shahprogrammer Thanks for the request. But I'm busy now and I think it takes some time to consider your request. Could you consider to send a PR to resolve this? I welcome your contribution.