Center Continue button instead of using left and right margins.

[mpatzer]

The intention of the code seems to be to center the Continue button. This centers it without making it stretch to the edges. This way the button is the size of the text (as one might expect) and can be customized (custom background color, etc.) without having a very wide button.

If #20 gets merged, I'll update this to use margin.centerXAnchor instead of self.centerXAnchor.

[mpatzer]

Resolved the conflict and updated to use margin.centerXAnchor.