Add support for argument definition about cli

[codegangsta]

This is an interesting idea, I would have to give some thought on how I might want this to work

[codegangsta]

I'm thinking something simple like:
App.ArgsHelp & Command.ArgsHelp

Will provide the functionality we would want. The default will be "[arguments...]" but can be overridden easily.

[codegangsta]

I added a commit to this but not quite sure how I feel about it. I'm going to tag this to add later as it is not a deal breaker (the template can easily be tweaked)

[shepmaster]

Can I customize [arguments ...] for a single command? It looks like the template is shared for all commands. As an example, I have these two commands with differing arguments:

cmd add name value
cmd rm name

I'm not sure how I could currently describe the two different sets of arguments.

[gravis]

This feature would be very helpful!

[matope]

👍 It would be really nice to have argument difinitions!

[AudriusButkevicius]

My pull request has some support of that, not sure if its enough though :)

[matope]

@AudriusButkevicius I've seen your PR. That looks very good.
It would be perfect for me if the arguments are printed in a help message.

[AudriusButkevicius]

Feel free to fork it.
Since I have help disabled for all of my commands, and all of my commands have subcommands, there is no way to list the arguments sensibly there.

[jszwedko]

Rolling this into #140