This update adds new functionalities to MacDown, and provides fixes to
bugs and usability issues, etc. Recommended for all users.

Major features
--------------

* Experimental plug-in architecture. MacDown can now accept plug-in
  bundles to add functionalities at runtime.
* Scripting support. You can now use AppleScript and JavaScript (JXA)
  to automate interaction with MacDown programmatically.

Resolved bugs
-------------

* Fixed a crash in the command line utility if filenames contain
  special characters.
* Fixed strange behaviours when toggling block syntaxes in a document
  caused by unintended integer overflow and underflow.
* Fixed an inefficient stack in the highlighter that causes CPU spikes
  when inline syntaxes are combined in a certain way.

Minor features and enhancements
-------------------------------

* MacDown now updates the document content when file is edited with
  an external application. This prevents you from accidentally saving
  an older version of the document.
* Convert lines to ordered list.
* The save dialog now always appends .md automatically when saving an
  untitled file. You can still change the extension if you want.
* The editor is now read-only when you hide it. Hotkeys still work
  (this is considered a bug that should be fixed in the future).
* The HTML document is now rendered through Handlebars.