Excludes `Image` objects when assembling plaintext content to write.

Fixes #24.

Obviously, this is the simple fix. When I looked at stopping `Image` from inheriting from `Paragraph`, I didn't get errors (and without this change, I still got the image hex in files). I'm still a little fuzzy on the finer points of the RTF spec and the reader's logic, so I probably need to clear that up before working on `Image`.