New hook for handling tokens before they are written

Proposed solution to #87.

Example usage:
```javascript
postscribe('#container', '<scr'+'ipt src="somescript.js"></scr'+'ipt>', {
    beforeTokenWrite : function(tok)
    {
        if (location.protocol !== 'https:' || !tok.tagName)
            return tok;
        
        if (tok.tagName == 'script' && tok.src && tok.src.indexOf('http:') === 0)
        {
            console.log('leaving script from insecure source unwritten');
            return null;
        }
        
        return tok;
    }
});
```
Old code (diff lines 348-349): add key, equals sign, quotation mark, value with escaped quotes, quotation mark.
New code (diff lines 359-360, 362-365): add key and for non-boolean attributes equals sign, quotation mark, value with escaped quotes, quotation mark.

So for non-boolean attributes (`typeof tok.booleanAttrs[key] == 'undefined'` is true) the quotes in the attribute value are escaped as before.

Or am I missing something?