You can now optionally configure a "no store" domain. When inbucket
receives a message destined for a user at that domain, it will accept
the message but not store it to disk. This allows the same instance
of Inbucket to be shared by people who wish to view email content and
those who want to load test.
- Refactor datastore such that we have a FileDataStore that implements
the DataStore interface.
- Add in missing SMTP configuration options: max recips, max idle, max
message size
- Add retention options to config
