Initialize `res` at the beginning of `_convert`. If the first converter raises an exception, then the `res` variable was not initialized and we got an error when checking `if res is not None`
* Work started moving converters to individual files.
* Significant cleanup and refactor.
* Moved everything to a packages subfolder.
* Added sample plugin.
* Added instructions to the README.md
* Bumped version, and added a note about compatibility.
