Feature Request: Surface the filename for embedded files

[leighmcculloch]

Embedded files store their filename as a parameter of the content-type and content-disposition. It would be handy if the parsemail package supported extracting them conveniently.

It's not particularly difficult to do with the mime stdlib package, so I can definitely understand if this is out-of-scope of this package and would be considered scope creep.

If the feature would be appreciated I'd be happy to contribute.

An example of how I'd implement would be something like this:
https://github.com/leighmcculloch/emlx/blob/487fb6c539137ec1d4b0c91bc2cb0118db7b4be3/main.go#L72-L78