Add markdown mailable and methods for embedding pre-made HTML components

[paulcsmith]

I'm not exactly sure how this will work but this is roughly what I want to be able to do:

## Welcome to MyApp

You can always [visit your account](#{UserProfiles::Show.with(@user.id})

#{mail_button("Log In", LogIns::New)}

It will send the markdown as-is for plain-text and will render it to HTML will ready made responsive styles. Would also allow having a mail layout that is responsive and customizable