Allow parsing of -Dkey=value style options

[Zollerboy1]

If I haven't overlooked something, it's currently not possible to define options in the style of make/gcc/clang definitions.

In theory it should be possible to define it like this:

@main
struct Foo: ParsableCommand {
    @Option(name: .customShort("D", allowingJoined: true), parsing: .singleValue)
    var defines: [String] = []

    public func run() throws {
        print(defines)
    }
}

This works fine for calling the executable e.g. like so:

$ foo -Dbar -Dbaz
["bar", "baz"]

However, when the value of the option contains an =, this doesn't work anymore:

$ foo -Dbar=baz
Error: Unknown option '-Dbar'

I'd guess that after seeing an equals sign the single "D" isn't considered anymore. It would be great, if that behaviour could be changed.

Of course, it would be possible to go an extra step and fully parse such key-value pairs into a dictionary. I could imagine a syntax like this:

@main
struct Foo: ParsableCommand {
    @Option(name: [.customShort("D", allowingJoined: true), .customLong("define")])
    var defines: [String: String] = [:]

    public func run() throws {
        print(defines)
    }
}

Where Option gets a new specialized init for Value == [String: Element] that then would work like that:

$ foo -Dbar=1 -D baz=2 --define qux=3
["bar": 1, "baz": 2, "qux": 3]