Add preprint information

You can see that crossref has started adding this info to their API. 
For both the published article. see the `has-preprint`field here:
https://api.crossref.org/works/10.1016/j.neuron.2019.05.022

as well as for the preprint original preprint. see the `is-preprint-of` field here:
https://api.crossref.org/works/10.1101/335703

This data should be made available in the JSON.