API endpoint to get user by email

[emschwartz]

I want to delete certain users from Plunk but I wasn't saving their Plunk ID when I created the contact. Since the API requires the ID in order to delete or update a contact, it would be very useful to have another endpoint to look up users by their email.

[m-collins]

This would be very helpful to support updating a contacts email address in situations where you don't want to store the plunk_contact_id

If a user updates their email address, you cannot:

• Use Track event (as this would create a new user)
• Use update contact (as either email or contact id is required)

[m-collins]

I have added a PR for this @driaug
#199

I can create a PR for docs for this too if a new endpoint is the right approach.

Alternatively, if you'd prefer to support the option to use email on the existing get_by_id endpoint, then I'll modify the pr.

However, given the naming of the current endpoint being explicitly for id, I opted for a new endpoint.

[driaug]

Closing this in preparation of next branch.