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'
Toggle "has-site-logo" class in the Customizer when adding or removing a logo. Fixes #9.
Toggle .has-site-logo <body> class in the Customizer when adding or removing a logo. Fixes #9.
Merge branch 'master' of github.com:Automattic/site-logo
merge
merge