Add issue template for bug reports

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,30 @@
+---
+name: Bug report
+about: Create a bug report to help us improve
+title: ''
+labels: ''
+assignees: ''
+---
+A clear and concise description of what the bug is.
+
+**To Reproduce**  
+Steps to reproduce the behavior, including, but not limited to:
+- Value of the environment variable `BOXES`
+- How *boxes* is invoked, including input text, ideally as a command which can be copy&pasted, for example
+
+      echo foo | boxes -s 11 -ac -d stone
+
+  If invoked via an editor plugin, give name of editor and plugin, and its configuration
+- If a custom box design is involved, its configuration
+- The output text produced by *boxes*
+- The error message your are getting, if any
+
+**Expected behavior**  
+What you *expected* to happen, for example the text that you expected *boxes* to produce
+
+**Screenshots**  
+If possible, add screenshots to help explain your problem.
+
+**Environment (please complete the following information):**  
+- *Boxes* version, e.g. `v1.3 (6d663cc)`
+- Your operating system and architecture, e.g. `MacOS X 64bit`, or `Windows 10`