Enh: provide alternative export format(s)

[sebbASF]

The mbox format is widely used and convenient but there are several variations in how From_ lines are handled. This can result in corrupted messages on import.

It would be useful to provide an export format that avoids these problems.

[sebbASF]

It should be easy enough to support the MMDF format which based on a single file [1]

The import tool does not currently support MMDF format, but that should easy to implement as it is supported by the Python mail package.

It would be harder to support formats which use a directory structure; the code would need to package the output files as an archive of some kind (tar, zip)

[1] http://www.tin.org/bin/man.cgi?section=5&topic=mmdf