Add buffered e-mail helpers

Not sure this is a good idea yet. Would need to have more use-cases in mind.

Some users of the library may want *not* to use a streaming API, for instance because they need to perform operations with multiple passes. We could expose APIs that let users process messages by loading them completely in memory, like almost all (or all?) other e-mail libraries do.

Of course this comes with the downside that whole messages (with their attachments) are loaded in memory.