Add support for notes and warnings

[AlexDaniel]

And maybe other things. See this list: https://github.com/fniessen/refcard-org-mode/blob/master/README.org#admonitions

Example:

#+begin_note
This is a useful note.
#+end_note

#+begin_warning
Be careful!  Check that you have...
#+end_warning

[AlexDaniel]

I noticed you use this:

#+HTML: <div class="notice notice-info">

#+HTML: </div>

So just changing

.notice-info {

to

.notice-info, .note {

everywhere in the css does the trick (well, kinda, you should also style .notice class).