Added option for custom headers

[kipcode66]

This PR adds an option to provide custom headers.

Relevant issues

Resolves #884

Contributor checklist

• [ ] Provide tests for the changes (unless documentation-only)
• [X] Documented any new features, CLI switches, etc. (if applicable)
  • [X] Server --help output
  • [X] README.md
  • [X] doc/http-server.1 (use the same format as other entries)
• [X] The pull request is being made against the master branch

Maintainer checklist

• [ ] Assign a version triage tag
• [ ] Approve tests if applicable