Enable extensions by default

This enables extensions such as:

* `autolink`: automatically recognize links
* `definition_lists`: PHP-Markdown style definition lists
* `no_intra_emphasis`: mid-word underscores don't screw up formatting
* `strikethrough`: surround words with `~~` to strikethrough
* `superscript`: superscript text using `^`
* `tables`: PHP-Markdown style tables
* `underline`: Use underscores for underline instead of emphasis

Not all of these options are available for each engine, and they can all
be disabled if needed in your `showoff.json` file. The AUTHORING docs
page has more information on the default options for each engine.
Merge pull request #188 from binford2k/update/load_more_extensions

Enable extensions by default
0.9.10 release

* Fresh coat of paint on the presenter view. This uses the CSS3 flexbox
  model, so it's much cleaner and will allow more UI work in the future.
* Slide menu in the presenter view now follows the current slide.
* Audience questions can now be dismissed by clicking on them.
* Smartphone remote view simplified, now with hamburger menu button.
* Enables more Markdown extensions as documented in AUTHORING.rdoc