Support to open the issue list page

Now, if the issue number is omitted with :OpenGithubIssue, it will open
the issue page. It also support open the issue page of specific
repository :OpenGithubIssue user/repository

Problem:
    :OpenGithubPullReq should open the pull request page
Update documents about opening-issue-list feature
Merge pull request #8 from haya14busa/support/open-issue-list

Support to open the issue list page if the issue number is omitted