Add View/Table support for ISIS adjacencies

Retrieves most of the stuff from 'show isis adjacency extensive' apart from adjacency log.

Key is a composite of interface and system-name which seems to be the most natural key to me. I hope I've used the composite keys implementation correctly - I went on a limb based on a quick read of the code.

Apart from that.. well, it works :)

Btw, thanks for #props =)