Add general constraint handling that can be the basis of depends/conflicts/either

[Fryguy]

From #130 (comment), @nanobowers wrote:

This seems like a specialization of a more general type of argument validation.

More specifically, the condition you are trying to satisfy with either: is max==1 and min==1. Should we entertain cases where min/max are not 1? conflicts: is kinda like max==1 and min==ANY

Was thinking something along the lines of:

validate min: <Integer/nil>, max: <Integer/nil>, args: <Array of Symbol>

[nanobowers]

@Fryguy taking a look at this.