Generic http authorization solution

[j3k0]

Instead of only supporting github.com private downloads, apm could use a more generic approach (as first mentioned in #38).

We could allow define per-domain authorization headers in .apm_config:

{
  "http_authorization": {
    "https://api.github.com/": "token ghp_myveryowngithubtoken",
    "https://private.domain.com/": "Basic YWxhZGRpbjpvcGVuc2VzYW1l",
  }
}

[marchbold]

The auth stuff is really just thrown in to get something working at the moment.

I believe we will want to implement a proper user authentication solution at the repository, this would then add a level of authentication into the repository queries which I'd see as being added into the repository nodes in the project file. Probably more likely to be some token or something than user/pass but that's the idea.

"repositories": [
        { 
            "url": "https://airnativeextensions.com/repository", 
            "credentials" : { "username": "", "password": "" }
        }
    ]

I see this as being essential for setting up and approving publishers as well.