Allow required array arguments, options, and flags

[MPLew-is]

Un-deprecates (but changes the semantics of) the initializers for an array value type without a default, forcing the user to specify at least one value from the command line.

The motivation behind this is to allow users of this framework to force a user to provide at least one argument, without having to provide custom error handling. An example of a command that uses this argument setup is cp, which has a variant for one or more source_file arguments:
cp [-R [-H | -L | -P]] [-fi | -n] [-apvX] source_file ... target_directory

This unfortunately does un-do the deprecations done In #193 and also is a source-breaking change in a similar vein as the one in #170 (changing the semantics of the case where no default is provided) and so will need to wait for the next minor release as per the documentation. I'm happy to sit on this and occasionally re-integrate master into it until you're ready for a new release, but figured I'd at least get it in for review.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[MPLew-is]

Also, does this need a documentation update? It's a purely additive change (when excluding deprecations) from what's on master, so it won't affect any existing documentation; however, I'm not sure if, how, or where you wanted to point this feature out, either.

[natecook1000]

@MPLew-is Thanks for this PR! This is a great addition, as zero-or-more and one-or-more are the primary expectations for array properties. Agreed that this will need to wait until 0.3.0.

For documentation, I think we'll want to add a section to guide 2 about how defaults work in general; this can be an example there.

[natecook1000]

@MPLew-is Ready to take this change — want to deal with the merge conflicts?

[MPLew-is]

Sure! I’ll push a resolution for them tonight, so you should be able to merge tomorrow morning.

[MPLew-is]

I've resolved the merge conflicts and extended this to Flag arrays (which I apparently missed the first time around), so this should be good to merge pending any review you have. I also took a first pass at updating the documentation, but it was a quick stab so may need some updating.

[natecook1000]

@swift-ci Please test

[natecook1000]

Tests and the doc additions look great, thank you!

[natecook1000]

@swift-ci Please test