Allow detailed pen styles to be set by structure.

I notice a lot of high school students drawing bar graphs.  This commit adds the ability to set more detailed pen styles by passing an object to the "pen" function.  So, for example, you can say:

<pre>
pen
  strokeStyle: red
  lineCap: 'butt'
  lineWidth: 100

fd 30
</pre>

And the result will be a bar 100 pixels wide and 30 pixels tall, instead of a big oval shape (the default lineCap for turtle drawings is 'round', which is nice for geometric shapes but not so good for bar graphs).