Add a command for closing fixed issues

[francis0407]

Describe the feature you'd like:

An issue might be fixed by a PR without being mentioned by the PR. If the assignee has the authority to close the issue (e.g. a committer of the project) but is not a member of the organization, we need a command for her/him to close that issue.

For example, the command could have arguments which are PRs closed it:

/close #1,#2,#3 

Or just close the issue without a PR (the issue might be a question):

/close

Do you have any ideas on how to implement it?(Optional)