Update markdown-mode to 1.8.1.

Major changes since 1.7:

- Improve syntax highlighting of preformatted text blocks to better
  distinguish them from list items with hanging indentation and nested
  list items.

- Match italic and bold text and inline code fragments across line
  breaks, within a single block, but prevent them from matching across
  blocks.

- Generally improve multi-line font lock by extending the search
  region to include full blocks.

- Fixed an issue, reported by Joost Kremners, where for multi-line
  lists, the position of the list marker was not being added to the
  list of possible indentation levels.

- Make indentation work when the previous line is not indented.

- Avoid a problem where indentation positions were getting skipped
  over when tab cycling.

- Fixed an issue when column 0 is the natural automatic indentation stop.

- Set tab width to 4, the natural tab width for Markdown documents.

- Stop announcing ���TAB��� in the minibuffer when tab is pressed.

(minskim)