Add new hook: c2s_post_auth_features

The c2s_post_auth_features hook can be used to extend the list of stream
features offered after authentication.
Offer CSI stream feature only if configured

Don't offer the CSI stream feature when mod_client_state isn't actually
configured to filter stanzas.  This makes sure clients won't send CSI
tags that end up being ignored.
Merge pull request #303 from weiss/no-csi-if-unconfigured

Offer CSI stream feature only if mod_client_state is enabled