Improve default behaviour for @arg(minElements)

[tfenne]

minElements currently defaults to one, so if you have a collection argument it requires at least a single value, and the only way to change this is to specify minElements=0.

I constantly do this and get tripped up:

@arg(doc="Optional thingies") val thingies: Seq[Thingy] = Nil

thinking that by assigning a default value to thingies I won't require any values on the command line.

I'd suggest that we make the following changes:

1. Change the default value for minElements to a symbolic value, I'm going to use -1 for the examples.
2. Change default behaviour so that if minElements == -1 and there is no default value supplied for an argument, interpret it as minElements == 1
3. If there is a default value supplied (even if that default value is an empty collection) interpret minElements == -1 as minElements == 0.

I think this is more in keeping with the behaviour for non-collection args that are required unless there is a default value, and keeps backwards compatibility.