Make length of dashes match length of label.

[nbeaver]

This is a purely cosmetic feature that makes the length of the dashes match the length of the headers. So instead of

---
Ingredients
---

it produces

-----------
Ingredients
-----------

[saxon-s]

Thank you for submitting this pull request. At this time we are actively migrating Gourmet to Python 3 and Gtk 3. I'm therefore going to hold off merging this PR into master until the new version is ready.