Show used values in "smart usage"

[kbknapp]

It would be nice to show actual values in the context sensitive usage statements:

i.e. current clap

$ prog --option val other.conf -a
error: -a isn't valid

usage:
    prog --option <some> <file>

What I'd like to see

$ prog --option val other.conf -a
error: -a isn't valid

usage:
    prog --option val other.conf

[kbknapp]

I ran into some snags while implementing this, values should be in the same order as they were originally placed in, but this requires some additional work. I'll have to play with it some more.