Allow SIP registration to upstream SIP provider

I use sipgate.de as upstream VoIP provider. 
To connect my asterisk to it, I have, additionally to configure upstream as a "SIP device", to set a register string in the [general] section in sip.conf.

Since I haven't seen another way to accomplish this, this patch allows me to do just that.
The additional parameter I set to $register_sip, defaulting to '' being empty. In that 
case the register => line in sip.conf will not be set. Following the pattern of other
variables used in that file.

Hope this is OK, or otherwise feedback appreciated.

cheers,
Sebastian