Add an option to trim whitespace from rendered HTML.

Add an option for trimming the generated HTML, which will do the following:
 * Remove all leading & trailing whitespace on every line
 * Remove all empty lines
 * Attempt to keep formatting inside < pre > < /pre > tags

This is fairly safe unless "white-space: pre;" is used in css for formatting. Since there is no way to detect that, you will have to keep trimming off in these cases.