Add concept of arguments

[peterbourgon]

Rarely do I want a simple <command> <opaque parameter string> structure. I'd like to register commands with some kind of argument spec, which could be validated and propagated to my handler function.

Concrete use case: register all of the following commands:

• foo —plain command as it exists now; trailing arguments passed as opaque string to handler
• bar <string> [string] —one string argument is required, everything after that is passed as an opaque string to my handler, which is (ideally) func doBar(string, string) but could be func doBar([]interface{}) or func doBar(interface{}...)
• baz {alpha,beta,delta} <integer> [string] — {alpha,beta,delta} being valid string tokens for that required argument (with tab-completion, natch), <integer> being a required argument that must successfully strconv.ParseInt, and [string] being an optional opaque string argument.

[raff]

Yup! I am thinking about that.

For now I have the "args" package, that splits a line into arguments (and I may extend it to do some processing of "options") and I am thinking if I should at least just split the command line into arguments given that I am ending up doing that for pretty much every command.

But I started by implementing exactly the same interface as python's cmd package, where you just get the full line.