Make settings in MIMEConfig configurable

[koen-serneels]

When mimepull is enabled MimePullMultipart is used which on its turn uses MIMEConfig for the configuration of "MIME message parsing and storing". However, a new instance of MIMEConfig is created hardcoded which results that the defaults of MIMEConfig are used, totally un-configurable. One of these defaults is the ~10mb treshhold to decide if an attachment is offloaded to disk or not.

As mimepull is already working via system properties, would it be possible to extend them and the allow the settings currently in MIMEConfig to be made configurable as well? Thanks!