Add support GRE/SIT tunnels. (#20)

Add support GRE/SIT tunnels. (#20)

This commit adds support for configuring GRE/IPIP/SIT tunnel interfaces as know
from previous versions of ifupdown. Currently only configuration checks for GRE
and SIT tunnels are implemented.

A tunnel interface configuration could look like this:

auto gre42
iface gre42 inet tunnel
        mode     gre
        local    198.51.100.1
        endpoint 203.0.113.2
	#
	# optional tunnel attributes
        ttl      64
        mtu      1400
	tunnel-physdev eth0
        #
        address  192.0.2.42/31
        address  2001:db8:d0c:23::42/64

auto he-ipv6
iface he-ipv6 inet tunnel
	mode sit
	endpoint 203.0.113.6
	local    198.51.100.66
	#
	# optional tunnel attributes
	ttl 255
	mtu 1466
	tunnel-physdev vrf_external
	#
	address 2001:db8:666::2/64

Signed-off-by: Maximilian Wilhelm <max@rfc2324.org>