Add --help flag for people looking for current flags

[jcc10]

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.

[jcc10]

Based off These lines

There are actually a bunch of flags that are just not written down for easy reference anywhere.

[jcc10]

Currently I have some stubby documentation at #1424 but if someone else has a better way of doing it please by all means go for it.