Attachments

[Terreii]

Is your feature request related to a problem? Please describe.
At the time of writing, there is no method of storing an encrypted attachment to a document.

Describe the solution you'd like
Add a methods to add, read, update and remove attachments.

Describe alternatives you've considered
In combination with #51 users could encrypt an Buffer/Blob and save them using PouchDB exposed through hoodie.store.db.

Additional context
This would require the creation of new APIs. They should follow Hoodies dreamcode philosophy.