Create issue templates

Add templates for bug, feature, and doc issues, based on GitHub's
standard templates.

Also create an `issue_template.md` file for blank issues of other types,
and configure the template selector to allow "blank" issues (which will
use that template).

.github/ISSUE_TEMPLATE/1-bug.md

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+---
+name: Bug report
+about: Create a report to help us improve WireViz
+title: "[bug] "
+labels: 'bug'
+assignees: ''
+
+---
+
+<!-- Please add a brief summary of your issue to the title above, after the
+     "[bug]" prefix -->
+
+**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):**
+- WireViz version: 
+- Graphviz version (`dot -V`): 
+- Python version: (`python -V`): 
+- Operating System and version: 
+
+**Additional context**
+Add any other context about the problem here.

.github/ISSUE_TEMPLATE/2-feature.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: "[feature] "
+labels: 'enhancement'
+assignees: ''
+
+---
+
+<!-- Please add a brief summary of your request to the title above, after the
+     "[feature]" prefix -->
+
+**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/3-doc.md

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+---
+name: Documentation issue template
+about: Help us improve the WireViz documentation
+title: "[doc] "
+labels: 'documentation'
+assignees: ''
+
+---
+<!-- Please add a brief summary of your issue to the title above, after the
+     "[doc]" prefix -->
+
+**What documentation page(s) does this involve?**
+Links to the relevant documentation files, e.g. [docs/README.md][].
+
+**Describe the problem**
+A clear and concise description of the documentation issue.
+
+**Describe your suggested improvement**
+A clear and concise summary of how the documentation could be better.
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.
+
+**Additional context**
+Add any other context about the problem here.
+

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true
+

.github/issue_template.md

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+<!-- Where appropriate, prefix your issue title with one of these
+     category prefixes, followed by a space:
+
+     [internal] - When suggesting code improvements that don't change
+                  any output.
+     [meta] - For issues about the development or contribution process.
+
+     For [bug], [feature], or [doc] issues, please use the corresponding
+     issue templates if possible, as they provide more guidance on what
+     information to include. -->
+