Rename loop from loop-$name.php to content-$name.php

The theme now uses the same naming convention as most other themes when
it comes to loop files.

To stay compatible with child themes, a cakifo_get_loop_template()
function has been created. It's a modified version of
get_template_part().

First it will try to get the old `loop-$name.php` then the new
`content-$name.php`. If neither of those exists, it will try `loop.php`
and `content.php`.