Add issue templates

This is the first stab at an issue template.
Their description should be generic enough to include them in most D repositories.
See also: dlang/project-ideas#43

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: Prio.3.Normal, Type.Bug
+assignees: ''
+
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**Platform**
+If applicable, provide informations about the platform the bug triggers on.
+*OS*: (Windows, Linux, Mac OSX, FreeBSD, Android, iOS, etc...)
+*Compiler*: (DMD 2.091.1, LDC 1.20.1, DMD@1897da, etc...)
+Any other tooling and version that is relevant (e.g. dub version, C++ compiler/version if the bugs concerns `extern(C++)`, etc...)
+
+**To Reproduce**
+A [Short, Self Contained, Correct Example](http://sscce.org/), and the expected behavior vs the actual one.
+Please provide code examples, command example, and error messages when applicable.
+E.g.
+```D
+void main () { /* Some code */ }
+``` 
+```console
+$ dmd -unittest -run test.d
+test.d(1): Error: cannot implicitly convert expression `a` of type `A` to `B`
+```
+Screenshot, links to repositories are also welcome, however bear in mind that anything that is self contained is less likely to disappear over time. Note that in order to reduce bugs, you can use [DustMite](https://github.com/CyberShadow/DustMite), which will reduce the bug for you. It is also available via `dub dustmite`.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: 'Suggest an idea for this project (see also: '
+title: ''
+labels: Prio.3.Normal, Type.Enhancement
+assignees: ''
+
+---
+
+**NOTE**
+For significant feature requests, please consider raising a [DIP](https://github.com/dlang/DIPs).
+If you're new to the language, consider [asking a question on the D forum](https://forum.dlang.org/group/learn) first.
+
+**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/regression.md

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+---
+name: Regression
+about: Change in behavior that broke your code or use case
+title: ''
+labels: Type.Bug, Prio.1.Regression
+assignees: ''
+
+---
+
+**Describe the regression**
+A clear and concise description of what the bug is.
+
+Please include :
+- The **previous behavior**;
+- When the change happened (which version of the compiler / library / tool);
+In order to find the timeline of a regression, you can use [Digger](https://github.com/CyberShadow/Digger).
+
+**Platform**
+If applicable, provide informations about the platform the bug triggers on.
+*OS*: (Windows, Linux, Mac OSX, FreeBSD, Android, iOS, etc...)
+*Compiler*: (DMD 2.091.1, LDC 1.20.1, DMD@1897da, etc...)
+Any other tooling and version that is relevant (e.g. dub version, C++ compiler/version if the bugs concerns `extern(C++)`, etc...)
+
+**To Reproduce**
+A [Short, Self Contained, Correct Example](http://sscce.org/), and the expected behavior vs the actual one.
+Please provide code examples, command example, and error messages when applicable.
+E.g.
+```D
+void main () { /* Some code */ }
+``` 
+```console
+$ dmd -unittest -run test.d
+test.d(1): Error: cannot implicitly convert expression `a` of type `A` to `B`
+```
+Screenshot, links to repositories are also welcome, however bear in mind that anything that is self contained is less likely to disappear over time. Note that in order to reduce bugs, you can use [DustMite](https://github.com/CyberShadow/DustMite), which will reduce the bug for you. It is also available via `dub dustmite`.