Add support for environment variables

[ohad]

Commands can expect environment variables to be set, and these are available to the handler.
The environment variables can be typed, and will be parsed before being handed to the handler.

(proposed by @eayus )

[mattpolzin]

Unlike arguments, these may more often apply to many or all commands. Not sure if/how that needs to factor in, but seems worth thinking about in the course of designing this.

[ohad]

Yes, I wonder whether these should be indexed in reverse (a node in the tree has an env. variable associated with it, and it's shared with its subcommands).