Imapbackup3

This a bug fix release [fixing 3 issues](https://github.com/DavidMStraub/imapbackup3/commit/8d4b796886df066667487142a4c878b9d9befa4a).

The new version brings major refactoring by removing old code and replacing it with more modern APIs. In particular, it uses the `mailbox` module for dealing with the local mailbox and the `email.message.EmailMessage` class for handling individual messages. This allows for new features but also requires Python 3.6 or newer.

New features

- The format of the local mailbox can now be chosen to be **Maildir** as alternative to mbox. There is a new `-m` switch on the CLI to choose it
- Using the Python API, a function `msg_filter` can be passed to the download methods that can be used to filter out or modify certain messages

Breaking changes

- Python 3.6 is now the minimum required version

This is the first usable release of `imapbackup3`.

This ports almost all of the functionality of the [original Python 2 script](https://github.com/rcarmo/imapbackup) to a Python 3 module with an almost compatible CLI interface, but also usable as a library by other packages.