github helper

## Features

##### 1. Duplicate issue button [Complete]

Although this feature is complete, it is fragile.
It will not work if the html design of github is changed.

As of 2019-03-01, it is working.

##### 2. Sort the issues in the project by assignee [Under development]

##### 3. Suppress Commit Links[Complete]

Github has `Autolinked references and URLs`.
However sometimes they are noisy.
So suppress comment links.

##### 4. Add outdated button for comment hiding [Complete]

Github can hide comment.
But it requires a lot of action.
So add `outdated` button for hiding comment.