Shell mode

[pschwartz]

Including variables and variable substitutions

[klmitch]

I had always intended to add a "shell" method to the ScriptAdaptor class, but I at first couldn't figure out a way to let the main function be called if any subcommands are defined. Now, however, I think I've discovered a way to do this: declare a subcommand named '' (empty string). This in combination with a "shell" method on the ScriptAdaptor should enable "progname" to invoke a shell if desired by the developer.

I will need to think a bit about how best to enable variables and substitutions. I think the shell needs to be able to pass around a context. I'm also thinking that an @options() decorator might be useful to set options on the implementing functions, such as "enable calling this if no subcommand is specified" or "pass this args directly"…