Add test for 'update-edge'
Add (failing) test for 'filter-nodes'
Fix 'filter-nodes' params ordering for consistency

Since all of the other graph API fns take the subject graph as the
first argument, 'filter-nodes' should behave the same way. This allows
easier "pipelining" of graph operations using the thread-first (->)
macro from Clojure core.

This fixes the failing 'filter-graph' test added in a previous commit.
Add test for (as yet undefined) 'subgraph' fn
Define 'subgraph' fn