Added an escape HTML option

I really needed an option to escape HTML, because I use jquery-truncate in a context where users can change text.

This pull-request added an option "escapeHTML". It replaces the "html" method with the "text" jQuery method. This way, you can truncate text with the right length and be HTML safe.

Feel free to do some comments about my work, I can change it if I did something wrong. Thanks for the previous work, this library helped me a lot.