Make stop-word lists configurable

This is a very interesting module (see #482106: Can tags be pulled from titles?), but to make it usable for other languages the stop-word lists should be made configurable.

I only had a quick look at the module code but found some instances where the languages are hard-coded (en and ru). A better approach might be if the module scanned the stop_words sub-directory and made available all languages with stop word lists there.

To this aim the stop word list format should be documented. Perhaps it could even be simplified from a full blown PHP array as it is now to a simple word list (comma separated or with each word on its own line).

Frank