twitter integration

(this is coded on top of n8han/pamflet#45 to avoid conflict)

Sample available at http://eed3si9n.com/tetrix-in-scala?color_scheme=monokai

Here's from the newly added docs:

> ### Twitter
> 
> Defining `twitter` property with any value will pop up a twitter button
> when some text is selected on any page. The button will compose a tweet including
> the selected text, the property, and the URL.
> The value `show` would display the button, but will not be included in the tweet.
>
>     twitter=#pamflet