Extension about .from() and .to() modulators

Current greskell supports use of `.from` and `.to` modulators for `.addE` step that take Traversal argument, but there are more uses about them.

- `.from` and `.to` modulators can be applied to `.path`, `.simplePath` and `.cyclicPath` steps.
- `.from` and `.to` modulators can also take step labels.

If you need this extension, just add a comment to this issue.
