Fix issue #271: Add support for <itunes:episodeType> on feed items

[neubig]

This pull request fixes #271.

The issue has been successfully resolved. The PR implements support for parsing the itunes:episodeType tag in RSS feeds. Specifically:

1. The decorateItunes method in the Parser class was updated to parse the itunes:episodeType tag.
2. A new test file itunes-episode-type.test.js was created to verify the functionality.
3. The package.json file was updated with the necessary dependencies and test configuration.
4. Tests were run successfully, confirming that the itunes:episodeType is correctly parsed for both 'full' and 'trailer' types.

This implementation allows users of the RSS parser to access the episodeType property for iTunes podcast feeds, enabling filtering based on episode types as requested in the original issue.

Automatic fix generated by OpenHands 🙌