Exporting pipeline as JSON and ability to recreate the pipeline from JSON

It would be nice to be able to export the pipeline as a json file and use them later on, example if you have to create pipelines for subsets of your training data based on some filter criteria.

## Expected result
Ability to export as json file
`tpot.export('customer_18345_pipeline.json')`

## Current result
Currently a pipeline can be exported as .py file
`tpot.export('customer_18345_pipeline.py')`