Log Message:
Update markdown-mode to 1.7.

Bug fixes:
    - Fixed several font lock issues.
    - Fixed a bug where pressing enter did nothing in longlines-mode.
    - Fixed a problem where emacs-snapshot would hang during
      markdown-enter and markdown-cycle.
    - Fixed an issue with auto-fill-mode and
      markdown-mode<E2><80><99>s indentat

New features:
    - Support filling of list items.
    - Allow customization of font-lock faces
    - Automatic indentation when pressing enter or tab (with cycling upon
      subsequent tab presses).
    - Generate real XHTML for previewing (thanks to Hilko Bengen for the
      patch). This fixes Debian bug #490865. The CSS stylesheet for XHTML
      output is customizable via markdown-css-path.
    - Better URL matching (protocols are customizable through
      markdown-uri-types
    - LaTeX syntax highlighting can be enabled or disabled using the
      customize system (markdown-enable-math).
    - Support for HTML comments (font lock, comment-dwim, etc.).
    - Support filling of definition lists.
    - Fill-paragraph no longer breaks lines inside of square brackets.
    - Add support level 5 and 6 atx-style headers.