add optional appcast stanza to DSL

Appcasts are a standard way for applications to publish
information about available updates via RSS.  Sparkle and
other update frameworks are built upon appcasts.

In a quick test, appcasts were found for 188/360 Casks examined.

This PR adds an optional `appcast` stanza, but does not add
`appcast` to any Cask or make use of the info in any way.  It is
intentionally left undocumented.