Update template-tags.php

I think giving the anchor the class will be much more flexible than using the image. You can still target the image specifically using .site-logo img, and you can target the wrapper.

The other option (to save breaking existing themes) would be to add a new class to the anchor (perhaps .site-logo-wrapper).
Give the site logo anchor tag a "site-logo-anchor" class. Props BinaryMoon, thomasguillot.
Merge branch 'BinaryMoon-patch-1'