Swift 5

Add support for Swift 5, and keep supporting Swift 3 and 4.

Support Swift 3 and 4

Enable Dual Compilation for Swift 3.1.1 and 4.1.2 (#5)

This change enables it to build/test for both Swift 3 and 4 at the same time. The code remains mostly Swift 4, but uses wrappers around the Swift 3 versions of functions to allow most of the Swift 4 code to continue to operate.

Swift 4. Repeated arguments. Counted Arguments.

Merge pull request #3 from kareman/Repeatedly

Update to Swift 4. Repeated arguments. Counted Arguments.

Improve error messages

• Improve error message when required argument is missing.
• Have error messages from each parser only print its own usage text and not the usage texts of the entire application.

0.4.1

If only "--" remains after parsing, remove it.