Housekeeping: Add GitHub issue templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: The plugin isn't working at all or shows wrong or unexpected behavior
+title: ''
+labels: ''
+
+---
+**Frequent Issues**
+
+* **E117: Unknown function: ingo#...**: Have you installed the [ingo-library plugin](http://www.vim.org/scripts/script.php?script_id=4433) (or via [GitHub](https://github.com/inkarkat/vim-ingo-library)) as well, as documented in the _dependencies_ section of the readme and plugin help?
+* **Sudden problems after updating**: Did you check out the default _master_ branch? Unless you want to participate in feature development and alpha testing, I would recommend that you switch from _master_ to the _stable_ branch; this way, you'll only update to released versions that are (hopefully) better tested and documented.
+* **Neovim**: I don't explicitly consider nor test Neovim compatibility; my plugins are written for Vim. If a plugin can be made to work on Neovim with trivial changes, I wouldn't mind including them, but anything more involved should in my opinion be filed as a compatibility bug against Neovim (as its homepage proclaims: _Fully compatible with Vim's editing model and the Vimscript language._)
+
+**Describe the bug**
+
+_A clear and concise description of what the bug is._
+
+**How to Reproduce**
+
+_Detailed steps to reproduce the behavior._
+
+**Expected Behavior**
+
+_A clear and concise description of what you expected to happen._
+
+**Environment**
+ - Plugin version (e.g. stable version _1.10_) / revision _a1b2c3d4_ from the master branch
+ - Dependency versions (e.g. [ingo-library plugin](https://github.com/inkarkat/vim-ingo-library), or external tool versions)
+ - Vim 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_)
+ - Install method: (e.g. manually via Vimball or ZIP file, GitHub clone as pack plugin, Plugin manager _NAME_)
+ - Other plugins / additional context if you think this could be important

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an enhancement for the plugin
+title: ''
+labels: enhancement
+
+---
+
+**Motivation**
+
+_A clear and concise description of what currently is hard to do._
+
+**Request and Purpose**
+
+_A clear and concise description of what you want to happen. Possible fit criteria._
+
+**Alternatives**
+
+_A clear and concise description of any alternative solutions or features you've considered._