Add data attribute (data-is-bullet-footer) to footer

Hey there,

this is a simple PR. I just needed a way to select the footer via CSS, so I can hide it on some views. Our use case was that we're using wkhtmltopdf and we don't want the footer to show up on the PDF, but we certainly want it to show up when we're browsing the website with a regular web browser.

Thank you for your work on Bullet, it's a great gem!