Enhancement: Provide cleaner func to minimize memory

[jimidle]

When using the parser as a step in a set of functions, it is sometimes necessary to serialize the envelope. it would be nice to have a minimizer function that cleaned out a few things that have been parsed and are not needed after parsing for probably a lot of applications.

Of course, it is trivial to write an external func to do this, but a builtin would just be a bit cleaner.

What I need to do is:

I save the attachment contents once they are parsed and available in the envelope parts. I then nil the Content field but retain the metadata.
I also nil out the Content field for html and text parts as they have been decoded and I don't need that Content going over the wire.

As I said, easy to do outside the package, but a func called something like Cleanup() or Minimize, with maybe an option set to ask for certain operations might be useful to others?

Release or branch I am using:

github.com/jhillyerd/enmime v0.9.0