Variadic functions and flag.Value-typed parameters

This release adds support for subcommand-implementing functions whose parameter lists end in ...string as an alternative to []string.

It also adds support for flag.Value-typed parameters.

It also adds support for multiple leading hyphens in the Param.Name field; i.e., you can write "-verbose" or "--verbose" (or even "---verbose") and it will mean the same thing: a flag named verbose, which at runtime will match a command-line option named -verbose or --verbose (per the behavior of Go's standard flag package).