Add the ability to specify if a team member should be a maintainer via config

[wesley-dean-flexion]

As an organizational admin, a fail-safe against an organizational admin not having the ability to modify a team, I would like the ability for the tool to automatically promote users who match certain filters to have administrative access.

[wesley-dean-flexion]

The USER_FILTERS could include another field, owner or admin or whatever, we could set it so certain users who match certain filters are automatically given admin access. The GitHub Users API includes a site_admin boolean that would be a good match -- if someone has site_admin set to true, set their membership to be owner (or maintainer or whatever). Something like:

{
  "site_admin": {
    "allow": [
      "true"
    ],
    "admin": "true"
  }
}

[wesley-dean-flexion]

maintainer is the role