add staticOptions for expose express.static options pass in

[hirokith]

Issue #327 (Expose a way to pass params for express.static)

[sokra]

Thanks

[sokra]

Could you add it to the documentation too?

[hirokith]

@sokra OK, I'll add it
Done in https://github.com/webpack/docs/wiki/webpack-dev-server#api