Add --help flag for people looking for current flags

Sometimes you just want to see what flags a program has, the most common way to check it is the --help flag.
 
Text should be something like
```
Usage: exokit [Flags] <url>

Flags:
--help
Display's this text then exits.
--xr [mode]
Forces a xr mode. (Modes are: webvr, webxr, etc.)
```

That may not be a exhaustive list of flags or options for the flags.