Basic issue and pr templates (#45)

## Description of changes

<!-- What changes did you make -->
Add issue and pr templates

The bug template is based on nixpkgs bug template

## Relevant Issues

<!-- Eg. #43 -->
#42

.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.
+
+### Steps To Reproduce
+Steps to reproduce the behavior:
+1. ...
+2. ...
+3. ...
+
+### Expected behavior
+A clear and concise description of what you expected to happen.
+
+
+### Environment
+
+Please run `rustc --version` and paste the result.
+
+```console
+[user@system:~]$ rustc --version
+output here
+```
+
+If using Nix please run `nix-shell -p nix-info --run "nix-info -m"` and paste the result.
+```console
+[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
+output here
+```
+
+---
+
+#### Notify maintainers
+
+<!--
+Please @ people who are relevant to this issue
+-->

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: "Feature request"
+about: Suggest an idea for this project
+labels: 'enhancement'
+---
+
+<!--
+Thank you for suggesting an idea to make nufmt better.
+
+Please fill in as much of the template below as you're able.
+-->
+
+**Is your feature request related to a problem? Please describe.**
+Please describe the problem you are trying to solve.
+
+**Describe the solution you'd like**
+Please describe the desired behavior.
+
+**Describe alternatives you've considered**
+Please describe alternative solutions or features you have considered.

.github/PULL_REQUEST_TEMPLATE.md

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+## Description of changes
+
+<!-- What changes did you make -->
+
+## Relevant Issues
+
+<!-- Eg. #43 -->