This PR introduces a new struct field for the message parts: `del`
If the del flag is set to `true`, the msgWriter will ignore this part during the writing process.
Additionally, the `part` has now a `Delete` method that lets the user mark the part as deleted
This allows middleware to take further control of the Msg and is part of #107
# SUMMARY
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* Read errors: 0
* Files with copyright information: 45 / 45
* Files with license information: 45 / 45
Congratulations! Your project is compliant with version 3.0 of the REUSE Specification :-)