Add "Markdown mode"

A few implementation notes (so that I don't forget why things are
implemented this way):

- Q: Why do the scripts rely so heavily on the clipboard? Why can't they
  just type out the desired Markdown text? A: Some editors auto-match
  certain characters (e.g., brackets). If type a left bracket (for
  example), the editor might automatically insert a right bracket. Since
  that behavior isn't universal, we can't rely on it for our purposes,
  but we need to ensure that we're not adversely affected by it. By
  *pasting* in the new text, we avoid this problem.
- Q: Why are the ext scripts a mix of shell script and AppleScript? Why
  aren't they pure AppleScript? A: I tried to implement one of the
  scripts as pure AppleScript. Because AppleScript commands execute
  asynchronously, I couldn't get it to work. The script would try to get
  the contents of the clipboard before the Command+C keystroke had
  completed. I don't see a way to make AppleScript commands behave
  synchronously.
Display error if extension not found

Before executing an extension, verify that the extension command exists.
If it doesn't exist, display an OS X notification to alert the user of
the problem.
Use OS X alert (not notification) to report error

Using a modal alert feels more appropriate and idiomatic for reporting
this error.
Document Markdown mode in README
Merge pull request #6 from jasonrudolph/markdown-mode-redux

Add system-wide keyboard shortcuts for Markdown