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, the idea being that we could
test it out on selected Casks and discover how well we can
make use of it.

Example usage for `evernote.rb`:
```ruby
  appcast 'http://update.evernote.com/public/ENMac/EvernoteMacUpdate.xml'
```