Continued from #585 and #583.

• Implement linting of mulled container's existence.
• Work through seqtk based example for documentation.
• Support multi-container registry for biocontainers.
  • Setup a composite recipes path in auto-mulled (e.g. composite-recipes/).
    • Use mulled-build-files to monitor that directory as part of the Travis build process for that repository.
    • Implement a check for existing published images in mulled-build-files.
  • Implement mulled_register command to open a PR against composite-recipes/ for a given tool or tools.
• Redo mulled concept with a version 2 of the hash - that hashes just the package names in the repository name and then hashes the versions in image version followed by a build number.