Allow a simple 'unnumbered: True' attribute on links

[jbemmel]

https://github.com/ipspace/netsim-tools/blob/master/docs/addressing.md#unnumbered-interface-support works, but is more complicated than it needs to be imho

Instead of:

role: <some-custom-pool-with-unnumbered-flag-set>

why not simply:

unnumbered: True

[ipspace]

I wanted to have a mechanism where I could do network-wide change from numbered to unnumbered (use case was OSPF unnumbered), but of course we can add the unnumbered flag and a bit of logic around address allocation. Will do.

[jbemmel]

I can see the reasoning to support large scale networks, but sometimes people simply want to focus on a single link. Having both options would be great, thanks

[ipspace]

Implemented in #92 / #97