Add support for a built-in datetime binding

[mathewc]

Similar to our existing {rand-guid} binding which allows people to easily generate unique blob paths declaratively (e.g. path: "images/{rand-guid}") we should also support a datetime binding that allows people to bind to date/time strings.

This might look like {datetime:mm-dd-yyyy} allowing standard date/time format strings to be specified. So in your binding you can simply do:

{
  "type": "blob",
  "name": "myOutputBlob",
  "path": "outcontainer/{datetime:yyyy-mm-dd}",
  "connection": "function43dc41c29824_STORAGE",
  "direction": "out"
}

We should generalize this so the pattern is {name:format} allowing format strings to be specified. E.g. allow {datetime:yyyy-mm-dd} to be specified, or {rand-guid:G}.

[mathewc]

@SeanFeldman might also have benefited from this feature in his scenario