Change scoped plugins name handling

[hueniverse]

No changes to plugin registrations and dependencies (the name provided is matched as-is). When using server.expose() in a scoped plugin, the name used under server.plugins defaults to no scope (breaking change). This can be changed to keep the scope with a new third argument { scope: true } option, or rewrite the scope to 'scope__name' with { scope: 'underscore' }.