Force to use an extractor

Add a way to force an extractor to be used - e.g. `wget` instead of `youtube-dl`.

Start by locating how tooru discovers and selects extractors, then inspect the command-line option handling. Confirm how `wget` and `youtube-dl` are represented and review any existing tests or usage documentation. Done means a user can explicitly force the selected extractor instead of relying on automatic selection.

Written by the indexing model from the issue text.