use the public API in knitr to generate figure filenames in tufte_handout()

There are a few subtle issues in generating the figure filenames, and it is not safe to paste fig.path with label as the filename. Possible issues are:

- more than one figure in one code chunk;
- special characters in fig.path or label (such as spaces) which may fail LaTeX;

This will fix the second problem reported in #269.