Support for #+ATTR_HTML

[ghost]

In org-mode, the #+ATTR_HTML syntax is used to specify attributes for the next block.

Example:

#+ATTR_HTML: :class goorgeous
I like Rust better.

gives

<p class="goorgeous">
I like Rust better.
</p>

You can have multiple lines, and multiples attributes per line, like so:

#+ATTR_HTML: :alt Go is fine though.
#+ATTR_HTML: :width 300 :style border:2px solid black;
[[file:https://golang.org/doc/gopher/pkg.png]]

This is probably easier to implement than #19 (although both would be useful).
_{Also, thank you! Support for org-mode made me switch to Hugo ;)}

[curiouslychase]

I had a branch I was getting ready to PR into Hugo this week that fixed this, but it looks like @niklasfasching PR'ed a new org parser, so I'm closing all the issues here, deprecating and archiving goorgeous.