Add templates for issues and pull requests

.github/ISSUE_TEMPLATE.md

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+### Subject of the issue
+Describe your issue here.
+
+### Your environment
+* Operating System (name/version):
+* Python version:
+* coursera-dl version:
+
+### Steps to reproduce
+Tell us how to reproduce this issue. Please provide us the course name, and the
+specific information if possible.
+
+### Expected behaviour
+Tell us what should happen.
+
+### Actual behaviour
+Tell us what happens instead. If the script fails, please copy the *entire*
+output of the command or the stacktrace (don't forget to obfuscate your
+username and password). If you cannot copy the exception, attach a screenshot.

.github/PULL_REQUEST_TEMPLATE.md

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+🚨Please review the [guidelines for contributing](CONTRIBUTING.md) to this repository.
+
+## Proposed changes
+
+Describe the big picture of your changes here to communicate to the maintainers
+why we should accept this pull request. If it fixes a bug or resolves a feature
+request, be sure to link to that issue.
+
+## Types of changes
+
+What types of changes does your code introduce?
+_Put an `x` in the boxes that apply_
+
+- [ ] Bugfix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
+
+## Checklist
+
+_Put an `x` in the boxes that apply. You can also fill these out after creating
+the PR. If you're unsure about any of them, don't hesitate to ask. We're here
+to help! This is simply a reminder of what we are going to look for before
+merging your code._
+
+- [ ] I have read the [CONTRIBUTING](/CONTRIBUTING.md) doc
+- [ ] I agree to contribute my changes under the project's [LICENSE](/LICENSE)
+- [ ] I have checked that the unit tests pass locally with my changes
+- [ ] I have checked the style of the new code (lint/pep).
+- [ ] I have added tests that prove my fix is effective or that my feature works
+- [ ] I have added necessary documentation (if appropriate)
+
+## Further comments
+
+If this is a relatively large or complex change, please explain why you chose
+the solution you did and what alternatives you considered, etc.
+
+### Reviewers
+If you know the person who can review your code please add a @mention.