Improve ATTR_HTML keyword parsing

Org mode separates kvs not as initially assumed by whitespace (~ csv) but
rather at keywords (~ :\w+).

This is still not replicating Org mode behaviour though as I decided against
attributes ignoring multi-definitions. Instead we stack their
values (and those existing on the element) for certain attributes (class, style
for now).

e.g.
[[foo]]

would become <foo class="a"> in Org mode but becomes <foo class="a b"> with
go-org.

org/testdata/keywords.html

@@ -1,5 +1,5 @@
 <figure>
-<div class="highlight a b c" id="it">
+<div class="highlight a b c d" id="it">
 <pre>echo &#34;a bash source block with custom html attributes&#34;
 </pre>
 </div>
@@ -11,7 +11,7 @@ and <span style="text-decoration: underline;">multiple</span> lines of <strong>c
 and an image with custom html attributes and a caption
 </p>
 <figure>
-<img src="https://placekitten.com/200/200#.png" alt="https://placekitten.com/200/200#.png" title="https://placekitten.com/200/200#.png" style="border: 10px solid black"/>
+<img src="https://placekitten.com/200/200#.png" alt="https://placekitten.com/200/200#.png" title="https://placekitten.com/200/200#.png" style="height: 100%; border: 10px solid black;" id="kittens"/>
 <figcaption>
 kittens!
 </figcaption>

org/testdata/keywords.org

@@ -1,13 +1,14 @@
 
 #+CAPTION: and _multiple_
 #+CAPTION: lines of *captions*!
-#+ATTR_HTML: :class "a b c"
-#+ATTR_HTML: :id it
+#+ATTR_HTML: :class a b
+#+ATTR_HTML: :id it :class c d
 #+BEGIN_SRC sh
 echo "a bash source block with custom html attributes"
 #+END_SRC
 
 and an image with custom html attributes and a caption
 #+CAPTION: kittens!
-#+ATTR_HTML: :style "border: 10px solid black"
+#+ATTR_HTML: :style height: 100%; :id overwritten
+#+ATTR_HTML: :style border: 10px solid black; :id kittens
 [[https://placekitten.com/200/200#.png]]

org/testdata/misc.html

@@ -207,7 +207,7 @@ foo
 <span class="todo">DONE</span>
 <a href="https://github.com/chaseadamsio/goorgeous/issues/72">#72</a>: Support for #+ATTR_HTML
 </h3>
-<img src="https://golang.org/doc/gopher/pkg.png" alt="Go is fine though." title="https://golang.org/doc/gopher/pkg.png" width="300" style="border:2px solid black;"/>
+<img src="https://golang.org/doc/gopher/pkg.png" alt="Go is fine though." title="https://golang.org/doc/gopher/pkg.png" id="gopher-image" width="300" style="border:2px solid black;"/>
 <h3>
 <span class="todo">DONE</span>
 <a href="https://github.com/chaseadamsio/goorgeous/issues/75">#75</a>: Not parsing nested lists correctly

org/testdata/misc.org

@@ -60,8 +60,8 @@ or ~that~ foo.
 [[https://www.example.com][/this/ *is* _markup_!]]
 #+END_QUOTE
 *** DONE [[https://github.com/chaseadamsio/goorgeous/issues/72][#72]]: Support for #+ATTR_HTML
-#+ATTR_HTML: :alt "Go is fine though."
-#+ATTR_HTML: :width 300 :style "border:2px solid black;"
+#+ATTR_HTML: :alt Go is fine though. :id gopher-image
+#+ATTR_HTML: :width 300 :style border:2px solid black;
 [[https://golang.org/doc/gopher/pkg.png]]
 *** DONE [[https://github.com/chaseadamsio/goorgeous/issues/75][#75]]: Not parsing nested lists correctly
 - bullet 1