Show min and max amounts in help

[justinpobrien]

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Issue It's not immediately obvious what minimums and maximums are. This could be made more apparent.

Actual Behavior When running loop out -h the help text currently shows:

USAGE:
   loop out [command options] amt [addr]

OPTIONS:
...
   --amt value                   the amount in satoshis to loop out (default: 0)
...

Expected Behavior Would expect to see the min and max visible in help text alongside the default

OPTIONS:
...
   --amt value                   the amount in satoshis to loop out (default: 0, min: 250k, max: 60m)
...