Add bin args for `watch` and `serve` commands

[jmjoy]

For #245

So we can pass command line arguments to bin, such as cargo leptos watch -v -- --foo.

[benwis]

LGTM, although I'm not sure what the use case is

[zakstucke]

@benwis for me this is useful to pass server config args processed with clap, I often find them more ergonomic than env vars!

Would be great if this could get merged, currently using this fork :)