Add issue and pr templates

Author: wooorm

.github/issue_template.md

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+### Subject of the issue
+
+Describe your issue here.  Remember, this is _not_ a place to ask questions.  For that, go to [Gitter](https://gitter.im/remarkjs/Lobby)
+
+### Your environment
+
+*   Name and version of operating system
+*   Names and version of required packages
+*   Version of node and npm, or names and versions of browser
+
+### Steps to reproduce
+
+Tell us how to reproduce this issue.  Please provide a working and simplified example.
+
+🎉 BONUS POINTS for creating a [minimal reproduction](https://stackoverflow.com/help/mcve) and uploading it to GitHub.  This will get you the fastest support.  🎉
+
+### Expected behaviour
+
+Describe what should happen
+
+### Actual behaviour
+
+Describe what happens instead

.github/pull_request_template.md

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+<!--
+
+Read the [contributing guidelines](https://github.com/remarkjs/remark/blob/master/contributing.md).
+
+We are excited about pull requests, but please try to limit the scope, provide a general description of the changes, and remember, it's up to you to convince us to land it. If this fixes an open issue, link to it in the following way: `Closes #123`. New features and bug fixes should come with tests.
+
+-->