fix the falsely assigned link for the cases of linking by reference and no link

Similar to `issue #69`, the false hyperlinks are assigned when rendering **reflink** and **no link** which are automatically hyperlinked by browser at editing.  This problem won't happen in plain text mode. 

##### Syndromes

```
[label]: http://<a href="http://example.com" target="_blank">example.com</a> 
``` 
will be rendered into 
```
[label]: <a href="http://[example.com](example.com)" ...>http://example.com</a>
```

----------

```
[label]: <a href="http://example.com" target="_blank">http://example.com</a> 
``` 
will be rendered into
```
[label]: <a href="[http://example.com](http://example.com)" ...>http://example.com</a>
```

##### Fix
Fix is done by adding the pattern of link reference to the regex in function _convertHTMLtoMarkdown_ .