Combine C-c C-o and C-c C-w for opening links (wiki and web)
Fix docstring
Jumping (C-c C-j)

Use C-c C-j to jump from the object at point to its counterpart
elsewhere in the text, when possible.  Jumps between reference
links and definitions; between footnote markers and footnote
text.  If more than one link uses the same reference text, a
new buffer will be created containing clickable links to each
link using the given reference in the main buffer.
Small documentation fix
Create undefined references when jumping (and other reference jumping tweaks)

Renamed markdown-add-missing-ref-definition to
markdown-insert-reference-definition.  Made BUFFER argument
optional, defaulting to current buffer.  Removed REFCHECK buffer,
moving code to button definition.

Reverse links so they are listed in order of appearance.
Omit line number links for easier tab cycling between links.

Update markdown-jump documentation.

Update markdown-reference-goto-definition docstring.

Close reference window upon jumping.
Factor out reference and link buffer creation
Remove debug message
Refactor and improve reference checking and link jumping buttons

Make buttons clickable with mouse-1 by adding 'follow-link property.
Update help text to indicate that they can be activated with mouse-1
or RET.

New function markdown-use-buttons-p checks for availability of button
feature or library, rather than checking Emacs version number
directly.  Use this function as a predicate for all button insertion
and definition code.

Refactor link buffer naming and creation.  The two recently created
functions markdown-reference-check-buffer and
markdown-reference-links-buffer now prepare the buffer by erasing it
and setting up the keymap.

Add functions for inserting buttons, to clarify calling functions:
markdown-insert-link-button and markdown-insert-undefined-reference-button.

Rearrange some functions in a more logical order (in order of dependence).
Use line-[beginning|end]-position consistently instead of aliases
Include all link types in M-n/p navigation
Use regexp-opt to optimize URI protocol matching