Add issue templates for bug reports and feature requests (#38)

Add issue templates for bug reports and feature requests; enhance contribution guidelines.

Author: Vianpyro

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+---
+
+## Describe the bug
+
+- A clear and concise description of what the bug is.
+
+## To Reproduce
+
+- Steps to reproduce the behavior:
+
+1. Go to '...'
+2. Click on '....'
+3. Scroll down to '....'
+4. See error
+
+## Expected behavior
+
+- A clear and concise description of what you expected to
+  happen.
+
+## Screenshots
+
+- If applicable, add screenshots to help explain your problem.
+
+## Desktop (please complete the following information)
+
+- OS: [e.g. iOS]
+- Browser [e.g. chrome, safari]
+- Version [e.g. 22]
+
+## Mobile (please complete the following information)
+
+- Device: [e.g. iPhone6]
+- OS: [e.g. iOS8.1]
+- Browser [e.g. stock browser, safari]
+- Version [e.g. 22]
+
+## 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: ''
+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.