Get profile information using the Steam Web API

Steam doesn't support the standard OpenID profile downloaded by passport-openid so the profile returned was empty.

This pull request implements getting the profile using the Steam Web API instead, which actually returns useful data. It requires that users get a (free, immediately available) API key from Steam though. As with other passport strategies, people can specify ``profile: false`` to disable requesting profiles entirely, in which case a Steam API key is not required.

You'll want to bump the major version number before releasing an update since this is a breaking API change.