Yesod.Settings provides the typical definitions for AppConfig,
AppEnvironment, and loadConfig (read from YAML).
Yesod.Main provides a single defaultMain function which accepts your
usual withSiteArg function and runs via Warp calling the now provided
loadConfig to figure out what to do.
Note: Yesod re-exports Y.Settings and Y.Main -- This is probably not the
right thing to do since it would cause collisions with users not using
the provided functionality (including all existing users).
