Detect internal_ip interface from default gateway

Add the option to specify `interface='default_gateway'` to the
`internal_ip` segment, which displays the IP address of the interface
connected to the default gateway.

powerline/segments/common/net.py

@@ -101,15 +101,22 @@ else:
 			return 0
 
 	def internal_ip(pl, interface='auto', ipv=4):
+		family = netifaces.AF_INET6 if ipv == 6 else netifaces.AF_INET
 		if interface == 'auto':
 			try:
 				interface = next(iter(sorted(netifaces.interfaces(), key=_interface_key, reverse=True)))
 			except StopIteration:
 				pl.info('No network interfaces found')
 				return None
+		elif interface == 'default_gateway':
+			try:
+				interface = netifaces.gateways()['default'][family][1]
+			except KeyError:
+				pl.info('No default gateway found for IPv{0}', ipv)
+				return None
 		addrs = netifaces.ifaddresses(interface)
 		try:
-			return addrs[netifaces.AF_INET6 if ipv == 6 else netifaces.AF_INET][0]['addr']
+			return addrs[family][0]['addr']
 		except (KeyError, IndexError):
 			return None
 
@@ -130,6 +137,11 @@ Requires ``netifaces`` module to work properly.
 	#. ``teredo`` followed by number or the end of string.
 	#. Any other interface that is not ``lo*``.
 	#. ``lo`` followed by number or the end of string.
+
+	Use ``default_gateway`` to detect the interface based on the machine's
+	`default gateway <https://en.wikipedia.org/wiki/Default_gateway>`_ (i.e.,
+	the router to which it is connected).
+
 :param int ipv:
 	4 or 6 for ipv4 and ipv6 respectively, depending on which IP address you 
 	need exactly.

tests/test_segments.py

@@ -397,6 +397,29 @@ class TestNet(TestCommon):
 			interfaces[:] = ()
 			self.assertEqual(self.module.internal_ip(pl=pl, ipv=6), None)
 
+		gateways = {
+			'default': {
+				netifaces.AF_INET: ('192.168.100.1', 'enp2s0'),
+				netifaces.AF_INET6: ('feff::5446:5eff:fe5a:0001', 'enp2s0')
+			}
+		}
+
+		with replace_module_module(
+			self.module, 'netifaces',
+			interfaces=(lambda: interfaces),
+			ifaddresses=(lambda interface: addr[interface]),
+			gateways=(lambda: gateways),
+			AF_INET=netifaces.AF_INET,
+			AF_INET6=netifaces.AF_INET6,
+		):
+			# default gateway has specified address family
+			self.assertEqual(self.module.internal_ip(pl=pl, interface='default_gateway', ipv=4), '192.168.100.200')
+			self.assertEqual(self.module.internal_ip(pl=pl, interface='default_gateway', ipv=6), 'feff::5446:5eff:fe5a:7777%enp2s0')
+			# default gateway doesn't have specified address family
+			gateways['default'] = {}
+			self.assertEqual(self.module.internal_ip(pl=pl, interface='default_gateway', ipv=4), None)
+			self.assertEqual(self.module.internal_ip(pl=pl, interface='default_gateway', ipv=6), None)
+
 	def test_network_load(self):
 		def gb(interface):
 			return None