support init repo from fork

[anGie44]

Closes #43

• Changes:
  • support forking as an additional method of creating a new repo with fork_from_repository and fork_into_organization optional attributes
  • fork_from_repository: to be provided in the format OWNER/REPOSITORY
  • fork_into_organization: to be provided if user wants to override default behavior of creating the new forked repo in the authenticated user's account

Output of acceptance tests:

--- PASS: TestAccGithubRepository_createFromForkForOrg (10.23s)
--- PASS: TestAccGithubRepository_createFromForkForUser (18.39s)