Added OpenHolidays API

[fstueber]

• [x] My submission is formatted according to the guidelines in the contributing guide
• [x] My addition is ordered alphabetically
• [x] My submission has a useful description
• [x] The description does not have more than 100 characters
• [x] The description does not end with punctuation
• [x] Each table column is padded with one space on either side
• [x] I have searched the repository for any relevant issues or pull requests
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[marcelscruz]

Thank you!