Added support for seek() and truncate()

Added support for seek() and truncate() methods on FileWriter and also improved write() support according to the File API specification based on code from FileWriter polyfill (https://github.com/piranna/ShareIt/blob/master/js/webp2p/polyfills/FileWriter.js) at ShareIt! project (also developed by myself). Since I started the polyfill using your implementation as basis, I think it's fair to return my improved and fixed version... :-)