Add SerpApi

[hilmanski]

Similar to SearchApi, we offer 100 free credits every month to access all of our APIs.

• [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
• [X] Any category I am creating has the minimum requirement of 3 items
• [X] All changes have been squashed into a single commit

[marcelscruz]

Thank you!