Added ToJSON and FromJSON instances

Hi,

This adds ToJSON and FromJSON instances to the Markdown newtype, so that it can be used in persistent entities with the JSON flag:

https://github.com/yesodweb/persistent/wiki/Persistent-entity-syntax#json-instances

As Aeson is already one of the dependencies on Yesod, I don't believe this as any major impact.

What do you think?
Cheers,
João