Footer improvements

[rghiorghisor]

I made 3 changes to the footer. Let me know what you think:

• Updated the twitter logo with the X one.
• Added the "Powered by" text before the theme text
• Added the possibility to extend the footer. The theme user needs to add a file (partials/extend_footer.html) in which he can add extra functionality. I myself needed this to add the functionality of zooming images, which involved adding js scripts to the footer.

[humrochagf]

I prefer to keep it without the "Powered by" for the theme credit not take more space than it already takes 🙂