Autolinks were not being displayed as such in the editor, changed
'linkhref' to 'link' in line 72 to fix it.

Added support for GFM double bracket links. '[[link]]' is now displayed
correctly as a link by the editor.

Added coloring support for latex formulas, usage of
- "$formula$",
- "$$formula$$",
- "\( formula \)", and
- "\[ formula \]"
will now be highlighted with the same coloring used for strings.

Closes #560

Closes #535

And highlight non-builtin symbols as "variable".

Closes #537

Closes #525

Issue #520

Issue #517

This is probably the first and only commit where
I deliberately *add* trailing whitespace (to show
off the feature here).

Closes #502

1) "colgroup" is not to be closed by another "colgroup" (there are other conditions in spec).
2) recursive self closing is allowed; example:
   <optgroup>
    <option>
   <optgroup>
3) selfclosers should also grab context; example:

   <optgroup>
    <option>
   <optgroup />

Closes #494

New list is taken from http://www.w3.org/TR/html-markup/syntax.html#void-element
Plus obsolete "frame" tag.