Don't add Geary-flavored markdown to plain text messages
Fixes #201 (closed)
If we add infer that a plaintext message should have links, bold, italic, or underline in certain places, we can put them in for display, but we should not modify the text content of the message which the author has personally formatted.
By the way, when composing a plaintext message in reply to a combined HTML/plaintext message, we should quote the plaintext version in the reply. And when replying to an HTML-only message, we should apply the same markdown formatting to it that we apply when composing a message.