Update HTML preview when converter is changed

Automatically update the HTML preview after the markdown
converter is changed in the options dialog.
Supported markdown extensions

Add method to query a MarkdownConverter implemention for supported
converter options.
Disable unsupported extensions in menu

Disable all unsupported Markdown extensions in the menu.
Merge pull request #125 from cloose/feature/improve-hoedown-integration

Improve integration of multiple markdown converters