Could the setting and requiring value delimiter automatically reflect in the help messages?

[zolnov]

Rust Version

• rustc 1.20.0 (f3d6973f4 2017-08-27)

Affected Version of clap

• clap 2.26.2

Expected Behavior Summary

Set some required delimiter:

.arg(
            Arg::with_name("volume")
            .short("v")
            .long("volume")
            .value_names(&["SOURCE","DESTINATION"])
            .value_delimiter(":")
            .require_delimiter(true)
            .takes_value(true)
     )

Help could show it:

-v, --volume <SOURCE>:<DESTINATION>

And if I run the code with space delimiter, clap could warn me to use ":" instead!

Actual Behavior Summary

Helps shows space delimiter:

-v, --volume <SOURCE> <DESTINATION>

Running with space delimiter it gives error, without the correct hint about what went wrong:

./prog -v a b
error: Found argument 'b' which wasn't expected, or isn't valid in this context

USAGE:
    prog --volume <SOURCE> <DESTINATION>

It gives error but does not show me to use ":" between a and b to get the correct behavior.

[kbknapp]

Yes it could. This would be a quick and easy PR if someone is interested!

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