New Command Parser Model

[jvalkeal]

Existing CommandParser was a rewrite in 2.1.x which turned out to be not so great implementation. It does its job but was more or less brute force type of scannerless parser.

To support missing features like option groups, double dash, etc it'd be beneficial to reimplement new parser from ground up having proper lexer, ast, parser concepts in it which would give better foundation for future needs.