core: expose IRC network name at bot.server.network

Return IRC network name parsed from `RPL_ISUPPORT`, if present, else
return None.

sopel/bot.py

@@ -64,12 +64,17 @@ class _ServerInfo(object):  # Eventually this can become a Python 3 data class
     hostname: A string containing 'servername' from RPL_MYINFO
 
     _isupport: A dict containing parsed values from RPL_ISUPPORT.
+
+    network: A string with the IRC network name from RPL_ISUPPORT.
     """
     def __init__(self):
         self.capabilities = {}
         self.hostname = None
         self._isupport = collections.defaultdict(lambda: None)  # Will be falsy when parameter not in RPL_ISUPPORT
 
+    network = property(lambda self: self._isupport['NETWORK'])
+    """Return the name of the IRC network that the client is connected to."""
+
 
 class Sopel(irc.Bot):
     def __init__(self, config, daemon=False):