fix performance issue in HtmlToPlainText#convert_to_text

If an empty link <a href="whatever"></a> is followed by more text, the
regular expression would take ages to complete its matching.

We originally found this issue when we found a background job that had
been running this regular expression at 100% CPU for 6 days.
Merge pull request #8 from codekitchen/html_to_text_performance

fix performance issue in HtmlToPlainText#convert_to_text