Added support to send Console error logs to Loggly

Added a configurable field to the library to send console errors to Loggly. Default value is true.
If someone assigns an event handler to `window.onerror` before our script loads, this will overwrite it.

To be safe, we should probably do the following:
*Wait until a user explicitly sets `setConsoleError = true` before overriding `window.onerror` (so basically move window.onerror assignment into setSendConsoleErrors). If setConsoleError is never specified, it's defaulted to false.

*Update readme to explain new proprty (I can help with that if you want)

```html
<script type="text/javascript" src="/js/loggly.tracker.js" async></script>
<script>
  var _LTracker = _LTracker || [];
  _LTracker.push({
    'logglyKey': '8c518f97-e3e0-4bfb-a8ed-582d084a5289',
    'setConsoleError': true
  });
</script> 
```

*Also, just to be safe, we could also save a reference to window.onerror before overwriting it, then calling that as well so we reduce the risk of breaking functionality on a clients page in case they don't realize some library depends on window.onerror as well. So for example, you could do something like this:

```javascript
var _onerror = window.onerror;
window.onerror = function(msg, url, line, col) {
    if (tracker.sendConsoleErrors) {
        tracker.push({
            category: 'BrowserJsException',
            exception: {
                message: msg,
                url: url,
                lineno: line,
                colno: col,
            }
        });
    }
    if (_onerror && typeof _onerror === 'function') {
        _onerror.apply(window, arguments);
    }
};
```