Add a clear button the to jsonlint.com page

[hugobyrne]

Hi,

Json lint is a very useful app.

Could you add a "Clear" button which removes the json text and resets the focus back to the input area. At the moment you have to drag and select all and then hit delete to remove the existing text.

thanks!

[teobais]

I've created a Pull Request for this, if you don't mind.