added support for convert options

Hi,
I have added an option to allow custom formatting of data, e.g. I need to convert boolean values to 0 and 1 instead of true/false. Also undefined doesn't have toString method, as a result toXml fails with an error exception.
And the last part is '*' case which allows to intercept all calls of toString, would be good to have some sort of a context, like a path to the current element, but for me it works quite fine as is, my only need was boolean and undefined values.