Add support of Opam package manager

[CameronEzraJones]

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Addresses the Opam request in issue #2043 . I thought I'd try tackling something a little meatier...

Methods implemented

• #project_names
• #project
• #mapping
• #versions
• #dependencies
• #recent_names
• #install_instructions
• #formatted_name
• #package_link
• #download_url
• #documentation_url
• #check_status_url

Constants implemented

• HAS_VERSIONS - false
• HAS_DEPENDENCIES - false
• BIBLIOTHECARY_SUPPORT - false
• BIBLIOTHECARY_PLANNED - true
• URL - https://opam.ocaml.org
• COLOR - #3be133

When I was working on this, it returned a little over 1500 projects, including projects that are being regularly maintained. Because of this, I believe that tracking this package manager aligns with the vision of Libraries.io

Adding versions and dependencies in the future is doable, but it would require a great deal of complex html extraction that I wasn't comfortable tackling right away.

Let me know if you have questions. Thanks!

[mbacarella]

Is there something blocking this from being accepted? Would be nice to have opam support!