extend support forks

If we have forked repo we still use fork issues, but we introduced new
parameter g:github_upstream_issues. By default it is 0 (disabled). If we
want to use issues from source repo (from what repo we forked) we should
set this setting to 1.

Reference: https://github.com/jaxbot/github-issues.vim/issues/20
Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>