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.

Fixes #126