Enhancement: Create Reader for the []byte of Content

[jimidle]

It would be useful to have a helper func something like ContentReader() the creates a reader from the []byte of Part.Content. It's not difficult to do this of course, but this would be an easy to provide helper function so that the mechanics can be encapsulated.

Release or branch I am using:

github.com/jhillyerd/enmime v0.9.0

(Please attach a sample message if you feel it will help reproduce the issue)