Streaming support for load & dump

Adds support for reading sources that have multiple edn elements and writing multiple edn elements to a destination.

Adds a new file _parsley.py which has the necessary evil to make this work.  I've also submitted that as a patch to parsley: https://github.com/python-parsley/parsley/pull/45

This depends on the fix to tagged values: https://github.com/dreid/edn/pull/16