Feature: Add the ability to save internal state

[daxbert]

The current implementation doesn't allow the internal state to be saved and later restored.

I'm proposing adding the following to TDigest:

• load function ( takes object typically achieved via JSON.parse )
• traverse function ( to traverse the centroid binary tree )
• _insert_centroid ( to allow direct insertion )

"saving" will be accomplished via JSON.stringify ( tdObject )

I'll be forking this repo and doing the above shortly.