Allow changing styles within wrapped text

This adds a new option named `continued` to `text`. If you set it to true, the wrapping state will be left in place so the next call to `text` will pick up where it left off.

Using this you can switch styles like `fillColor` and `font` and then continue outputting text within the same flow.

[Here is a demo of the feature](http://ef4.github.com/pdfkit-www/continued.html) (running in-browser with pdfkit-www, doesn't work on IE).