Add issue templates for bug and feature request

.github/ISSUE_TEMPLATE/bug.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: Bug report
+labels: bug
+
+---
+
+<!--
+NOTE: The issue list is not the place for general questions.
+Please consider using the mailing list instead:
+https://groups.google.com/a/flatironinstitute.org/forum/#!forum/triqs
+-->
+
+### Prerequisites
+
+* [ ] Put an X between the brackets on this line if you have done all of the following:
+    * Checked that your issue isn't already filed: https://github.com/issues?utf8=%E2%9C%93&q=is%3Aissue+user%3Atriqs
+
+### Description
+
+[Description of the issue]
+
+### Steps to Reproduce
+
+1. [First Step]
+2. [Second Step]
+3. [and so on...]
+
+**Expected behavior:** [What you expect to happen]
+
+**Actual behavior:** [What actually happens]
+
+**Reproduces how often:** [What percentage of the time does it reproduce?]
+
+### Versions
+
+You can get this information from copy and pasting the output of
+```bash
+python -c "from app4triqs.version import *; show_version(); show_git_hash();"
+```
+from the command line. Also, please include the OS and what version of the OS you're running.
+
+### Additional Information
+
+Any additional information, configuration or data that might be necessary to reproduce the issue.

.github/ISSUE_TEMPLATE/feature.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: Feature request
+labels: feature
+
+---
+
+<!--
+NOTE: The issue list is not the place for general questions.
+Please consider using the mailing list instead:
+https://groups.google.com/a/flatironinstitute.org/forum/#!forum/triqs
+-->
+
+### Summary
+
+One paragraph explanation of the feature.
+
+### Motivation
+
+Why are we doing this? What use cases does it support? What is the expected outcome?
+
+### Describe alternatives you've considered
+
+A clear and concise description of the alternative solutions you've considered.
+Be sure to explain why the existing customizability of app4triqs isn't suitable for this feature.
+
+### Additional context
+
+Add any other context about the feature request here.