Create new issue templates (#934)

* Create new issue templates

* style -> design

* Apply suggestions from code review

Co-Authored-By: Hugo van Kemenade <hugovk@users.noreply.github.com>

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. Take this file '...'
+2. Run *Black* on it with these arguments '....'
+3. See error
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Environment (please complete the following information):**
+- Version: [e.g. master]
+- OS and Python version: [e.g. Linux/Python 3.7.4rc1]
+
+**Does this bug also happen on master?**
+To answer this, you have two options:
+1. Use the online formatter at https://black.now.sh/?version=master, which will use the latest master branch.
+2. Or run *Black* on your machine:
+    * create a new virtualenv (make sure it's the same Python version);
+    * clone this repository;
+    * run `pip install -e .`;
+    * make sure it's sane by running `python setup.py test`; and
+    * run `black` like you did last time.
+
+**Additional context**
+Add any other context about the problem here.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.

.github/ISSUE_TEMPLATE/style_issue.md

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+---
+name: Style issue
+about: Help us improve the Black style
+title: ''
+labels: design
+assignees: ''
+
+---
+
+**Describe the style change**
+A clear and concise description of how the style can be improved.
+
+**Examples in the current *Black* style**
+Think of some short code snippets that show how the current *Black* style is not great:
+```
+def f():
+    "Make sure this code is blackened"""
+    pass
+```
+
+**Desired style**
+How do you think *Black* should format the above snippets:
+```
+def f(
+    ):
+    pass
+```
+
+**Additional context**
+Add any other context about the problem here.