Improved CLI extensibility

The code from the runDubCommandLine was moved to some other structs. The new
CommandLineHandler structure takes care of the CLI state and commands, which
allows DUB commands to be extended in other projects.

The CommandArgs class now has new `hasAppArgs` and `appArgs` methods which
allows you to get the app arguments. In addition, the `extractAllRemainingArgs`
method will return all unused arguments and the app arguments.