Add passthrough support

As originally proposed and discussed in flapjack/flapjack#553, this PR adds passthrough support to GLI. 

Passthrough is a method to tell GLI to stop parsing arguments and just call the action as-is. 

The motivation for this was twofold:

- Calling the command with --help invoked the GLI help, not the native help provided by whatever command you're calling.
- Elimination of double handling of arguments and options. Without this, the developer needs to replicate all the arguments and options of the command being called within the GLI command definition.

There is also a minor whitespace cleanup. 
Can you promote `get_action` (see https://github.com/auxesis/gli/blob/gli-2/lib/gli/command_support.rb#L154 ) to public instead of this?

That method is slightly different in that it will always return a callable thing, so it handles the case where a command is just grouping subcommands but has no action itself (which seems like what you'd want here, too)