Generate plain text version from the HTML version

[doup]

Right now Renderer::htmlToPlain() returns the input html. Generate a plain text version.

[eillarra]

A good mailing service (Mandrill, for example) generates the TXT version from the HTML version. Do we really need to include the option in the bundle?

I think that Renderer::htmlToPlain() is not necessary and that users can add the TXT, if they want, using the SwiftMailer methods.