Implement command: zigup run

[iddev5]

Implements the command with the signature zigup run <version> <args>... or zigup run <version> -- <args>

This PR implements the concept of positional arguments into zigup. This is because commands like zigup run 0.8.1 --help for example, would run --help command of zigup as opposed to zig 0.8.1, so the solution was to use positional arguments to reliably pass arguments into the compiler

Doesn't work for zigup run master

Closes #23

Open question:

1. Should master be allowed? If so then same way as zigup default?
2. Is this way to pass args ok? The other solution is to pass all the arguments as a single string, that should be simple to implement, as per desired.

[marler8997]

I think on the initial implementation we can leave out the positional argument logic. If it becomes an issue then we can address it.

[marler8997]

I realize I should maybe clarify what I'm looking for here.

In the argument parsing loop, we should add a special case that checks whether the first non-option argument is "run". If that's the case, we stop argument parsing and immediately call a function that implements the "run" command with the rest of the arguments.

P.S. Should master be allowed? Not sure, I'm fine leaving it for later though.

[iddev5]

Its done. To be honest, I didn't implemented it in terms of this in the first place because it felt hackish to me, and that such a special case would not work in case of a real argument parser, and thus be a bit counter-techincal. But its okay, it works.

Leaving master for now.