Add Test-GitHubOrganizationMember

[HowardWolosky]

• Added Test-GitHubOrganizationMember to check to see if a user is in an organization.
  • No UT was added for this, because we would only be able to test for the negative case since the API can't create new users.
• Updated Get-GitHubTeamMember to optionally work directly with a TeamId (instead of it always implicitly looking it up when being given the TeamName).
  • Similarly for Teams -- more of the API set needs to be built out before this can be individually tested.