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/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