Add contactPoint to extensions data

[odscjames]

This spun off from #85

• [ ] Add to https://github.com/open-contracting/standard_extension_template/blob/master/extension.json and the appropriate JSON Schema. (This way extension authors can edit these values themselves - changes probably don't require approval.)
• [ ] Change compile scripts in this repository to read values and add them to generated output.
• [ ] Update documentation on creating new extensions

Should the format allow multiple authors, something like:

{
    "authors": [{
            "name": "James",
            "email": "james@example.com "
        },
        {
            "name": "Bob",
            "email": "bob@example.com"
        }
    ]
}

Fields fully optional so existing extensions aren't suddenly marked as broken. Also do we want to support people giving a name but no email?

[jpmckinney]

I prefer a single point of contact, as otherwise people may contact one and not another, leading to confusion, etc. Since we want a contact point, rather than e.g. author attribution, we could have, e.g.:

{
  "contactPoint": {
    "name": "J",
    "email": "j@example.com"
  }
}

[jpmckinney]

Noting that this issue likely belongs on https://github.com/open-contracting/ocds-extensions/issues, as the contact information is anticipated to be added to each extension, rather than centrally controlled by the registry.

[jpmckinney]

Moved to https://github.com/open-contracting/ocds-extensions/issues/78