Update issue templates

This adds two simple issue templates for bug reports and feature request.
Both templates are based on the default templates by GitHub and slightly
modified for Vim.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,32 @@
+---
+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.
+(Issues related to the runtime files should be reported to the each maintainer. Not here.)
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. Run `vim --clean` (or `gvim --clean`, etc.)
+2. Type '....'
+3. 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.
+
+**Environment (please complete the following information):**
+ - Version [e.g. 8.1.1234] (Or paste the result of `vim --version`.)
+ - OS: [e.g. Ubuntu 18.04, Windows 10 1809, macOS 10.14]
+ - Terminal: [e.g. GNOME Terminal, mintty, iTerm2, tmux, GNU screen] (If you use a CUI version.)
+
+**Additional context**
+Add any other context about the problem here.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,21 @@
+---
+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 [...]
+(If it is related to runtime files, please discuss with the maintainer.)
+
+**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.