Add support for blacklisting a contributor

[rithviknishad]

We may not want to track contributions of certain accounts. Add support for blacklisting github usernames, so that the scraper would ignore tracking activities of such users.

Example config:

plugins:
  github:
    githubToken: ******
    githubOrg: ohcnetwork
    blacklist:
      - invalid-email-address

Example: https://contributors.ohc.network/invalid-email-address/