Adding conditional wraps around two regexes in doAnchors, this should pr...

This patch wraps two regular replace()s in a safety check that should prevent the run away callback scenarios from occurring and not affect the replace().

Wrapping the regex in a safety check aslo keeps the regex code untouched, so it can be compared easliy with the "upstream" PHP impl.

To test before and after try:
```javascript

    var start = new Date().getTime();
    md.Markdown(str);
    var elapsed = new Date().getTime() - start;                                                                                                                                 
    console.log("run #" + i + " took: " + elapsed + "ms");
```