Introduce copy/paste functionality

[dionyziz]

When selecting multiple nodes, Ctrl+C (Cmd+C on Mac) should copy them and Ctrl+V (Cmd+V on Mac) should paste new ones that are of the same topology.

[dionyziz]

$10 bounty

[JustABiologist]

Submitted PR #62 for this bounty: #62 Verification: node -c site/js/editor.js; node-based smoke test for copying and pasting selected states and their internal transition. If accepted for the listed bounty, PayPal payout can be sent to stab.me.papi@gmail.com.