Added option for custom headers

[dxdxdt]

Difference/improvements over #885:

• Semantically identical to curl -H and curl --header ^1
• Empty headers are allowed (RFC 7230)
• Includes tests

Sorry :)

Relevant issues

Resolves #884

Contributor checklist

• [x] 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