feat: Allow custom serializer and deserializers for task

[kgritesh]

Fixes #21

[xuru]

FYI, this doesn't use the custom encoder/decoder in gen_unique_id which I ran into using this code.

[kgritesh]

@alanhamlett @xuru let me know if we need to use this custom serialiser in get_unique_id function. Also let me know if anything pending for this to be merged?

[xuru]

@kgritesh I took a different approach then passing in a call back for serialization/deserialization. I decided to do it like flask does it, and pass in the json encoder/decoder in order to serialize custom types. I created a pull request: #99