Add option to execute callback only once
Fix error in IE8- for empty elements

When an element has no content, `elem.innerText` will be falsey,
so the right hand side of the `||` operator is used.
Since `.textContent` is `undefined` in IE8 and before, the result
will not be a string, but `undefined`.
Later operations which except the variable to be a string will fail,
because `undefined` does not support any string operations.
Patched stripTags option - removes severe XSS leak

`stripTags` comes with a severe security hole.
The demo and rationale for the solution is posted at
https://rob.lekensteyn.nl/Countable/
Version bump
Fix a bug where options wouldn't be applied correctly