Add stun server for ip discovery and flag stun

[r4f4ss]

fix #176

new usage (reference usage):

$shisui --nat stun

or

$shisui --nat stun:<addr>

[GrapeBaBa]

@r4f4ss (nit) Could this feature have a test?

[fearlessfe]

It just create a STUN{}, It should be a place to get the real IP and set to the localNode

[r4f4ss]

It just create a STUN{}, It should be a place to get the real IP and set to the localNode

Added function main.doPortMapping that set address and port for localNode, reference implementation from:

shisui/cmd/bootnode/main.go

Line 153 in ee38628

func doPortMapping(natm nat.Interface, ln *enode.LocalNode, addr *net.UDPAddr) *net.UDPAddr {