[feature] better json deserialization support

[connectdotz]

There didn't seem to be a symmetric way to retrieve an object as written by textile.files.add(anObject, caption, threadId) for 'json' scheme.

1. it's great that we can pass the original object without stringify, but it is not clear how we can retrieve this object through existing API, I have tried textile.file.content and textile.ipfs.cat, both gave blob instead of the deserialized object.

2. once we have the new API that can indeed restore the data back to its original form, i.e. calling JSON.stringify() on the json string extracted from internal response blob, it will be great if textile can take an optional reviver like argument so developers can customize the deserialization accordingly, such as creating a Date object when encountering a ISO date string, among others.