Basic share mode implemented. Fixing #20.

- Added a basic share mode, fixing #20.
- I've also added a check to see if the browser supports localstorage when trying to save.
- Added a URL to the initial content to showcase the URL feature.

To edit a document, simply append #edit to the URL.

Example:
This is not editable: index.html#header#content

This IS editable: index.html#header#content#edit