fix accum propagation in do-dfs

- #:break and #:break: can now use the accum
- closes #15
add do-dfs tests

- test clauses that explicitly name vertices
- test with and without explicitly named accum
docs: add line breaks in do-bfs and do-dfs signature; closes #13
docs: fix typos in do-dfs and do-bfs; closes #14
docs: add-edge, add-directed-edge: fix default wgt val

- closes #12