Add Tag.attributes_escape/1

[msaraiva]

This PR adds a new function called attributes_escape/1 that will be used by the new template engine I'm working on. I'm not sure this function should be placed in the Tag module since neither tag nor content_tag use it. However, I believe they should use it for consistency. The problem is that, currently, the implementation sorts the attributes, which makes no sense for the new engine as it generates groups of attributes separately based on their types (static key/value, static key + dynamic value and dynamic key/value).

I can either move this function to someplace else or change the implementation of tag and content_tag to use it. The latter might break existing code if, for any reason, the order of the attributes was taken into account, for instance, in tests.

[josevalim]

We should document that, opposite to tag and content_tag, the attributes are not sorted.

[josevalim]

Just one comment and we can ship it!

[josevalim]

💚 💙 💜 💛 ❤️

[msaraiva]

@josevalim regarding accepting class: [active: path == "/"] as suggested by @wojtekmach in #276 (comment), I don't see why we shouldn't accept it. After all, the user can do stuff like ["user_#{id}": ...] on basically any function that accepts an atom or a keyword list. I don't see why we should treat this differently. Classes are, by nature, mostly not dynamic and when they are, the set of possible classes is still finite/predefined, not based on some infinite id. I actually never saw anyone using classes based on ids.