Includes the following updates:
- Simplified/reworked parser
- Support for RFC2231 header parameters
- Use Psr streams and decorators instead of built-in stream_filters and extensions (zbateson/stream-decorators and GuzzleHttp streams)
- Ability to use other character sets
- Dynamically generate streams when saving messages -- only generate a dynamic stream on modification to a message and simplify writing
In addition much code has been refactored and cleaned to be more maintainable, testable and stable.