Update issue templates (#667)

.github/ISSUE_TEMPLATE/bug_report.md

+---
+name: Bug report
+about: Report a bug in OMF
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+Describe the problem you are having and how to reproduce it. Include any other additional context that can help us troubleshoot.
+
+**Environment report**
+
+Run the command `omf doctor` and paste the output here. Example:
+
+```
+Oh My Fish version:   6
+OS type:              Linux
+Fish version:         fish, version 2.7.1
+Git version:          git version 2.17.1
+Git core.autocrlf:    no
+Your shell is ready to swim.
+```