Advertise in-band registration support.

Advertise in-band registration support.
Implement optional registration support advertisement according to
XEP-0077 sec8:

<features>
    <register xmlns='http://jabber.org/features/iq-register'/>
</features>
---
 plugins/mod_register.lua |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)