Added attribute to center button contents

[Jwildsmith]

Added a simple attribute to the StretchableButton widget to allow for centering of the button contents as the buttons expand.

• I left the default behavior not centered and the attribute false to maintain compatibility.
• I also updated the current library of auth buttons to work with this change.
• Tested within my own project and behaves like original package with new functionality
• Not sure if any documentation changes are needed as it is an easily found attribute for the widgets, open to discussion and revision of current documentation if needed.

Fixes #31

[dmjones]

LGTM, thanks for opening this. I've made a few minor tweaks and will merge this work in #39.