config: make server configurable

irc.freenode.net is hardcoded in the source code. This patch introduces
a new configuration key 'server' and updates the config and sample
config to have server pointing to 'irc.freenode.net' as previously.

Change-Id: I720e33b004ada6a8ec1949dc6857a3bc433299bd
Merge "config: make server configurable"