Label alignment settings

- fix hover-area miscalculation due to CanvasRenderingContext2D not
  using the appropriate font size and font family
- add label alignment support for svg

There is an issue in Firefox that when you zoom in/out, the hovered areas don't get removed. This issue should be fixed by previous pending pull requests, https://github.com/jacomyal/sigma.js/pull/459 and https://github.com/jacomyal/sigma.js/pull/454