Add functions to format attributes

[ryanve]

I'd like to see WP add functions for attributes. These would be extremely useful and resuable for theme devs and internally. I've developed very solid generic functions for this in ​phat and would be happy to make WP versions to the effect of:

wp_attrs( $array ) // converts array to attributes string
wp_attrs( $name, $value ) // formats attribute string
wp_attrs( $string ) // reformats attribute string
wp_parse_attrs( $string ) parses attributes into assoc array

Related: #23237

Here's how I think ​wp_parse_attrs() should be. It uses a primitive loop and is very fail-safe. I don't think regexp is viable due to complications from quotes (single, double, unquoted, JSON). ​DOMDocument is viable but too brittle.

I'll work up a wp_attrs() function similar in effect to ​phat\attrs() but specific to WordPress.

I added ​wphat.php in the ​phat repo called where I'm working on this. I think it'll be better to name the functions wp_attr and wp_attr_parse b/c both can handle 1+ attributes.

How do we want to encode complex (array|object) attributes? ​Currently it checks for the ~10 attributes that take tokens ([class], [media], ​etc.) and implode them with the correct glue. Otherwise it uses json_encode. Should we skip the check and go with the simple approach of using json_encode for any complex value? In either case I'll include a 'complex_attr_value' filter. I avoided esc_attr b/c it breaks JSON and b/c it double-encodes entities.