Add issue template for bug reports and feature requests

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report a problem with dhewm3
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!-- NOTE: Please check the existing bug reports first to find out if it has already been reported. In that case, just add a comment there with your additional information -->
+
+**Describe the bug**
+A short description of what the bug is.
+
+**Your System**
+- What **operating system** are you using, what CPU and GPU, which GPU driver version?
+- What **version of dhewm3** are you using? (Also: Official download? Package from Linux distro? MacSourcePorts? Did you compile yourself?)
+
+**To Reproduce**
+Describe how to reproduce the bug
+- if it happens while playing: What mod (base game, RoE, or some 3rd-party mod), which level, what do you do there to trigger the bug
+    - After opening the console (`Shift`+`Esc`) you can enter `map` to print the current map name and `where` to print the coordinates in the level
+- if it happens in the menu or similar: describe the steps to trigger the bug
+
+**Expected and actual behavior**
+A clear and concise description of what you expected to happen and what happens instead
+
+**Additional information**
+If applicable, add screenshots or a video to help explain your problem.
+Please attach [dhewm3log.txt](https://github.com/dhewm/dhewm3/wiki/FAQ#where-do-i-find-the-config-files-and-savegames-and-dhewm3logtxt)
+A **savegame** also often helps to more quickly reproduce a bug

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for dhewm3
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+<!-- NOTE: Please check the existing feature requests first to se if it has already been reported. In that case, just add a comment there with your additional information/ideas -->
+
+**Describe the feature you want**
+A clear and concise description of what the feature is supposed to do.
+Add screenshots or illustrations or whatever if you think it helps explaining.
+
+**What problem does it solve?**
+Describe what problem the feature would solve, or how it would improve the game.
+
+**Additional information**
+If you have ideas how it could be implemented, or what other open source projects to check out that already have the feature, or links with information about the feature or any other additional information, post it here.