Support more issue-reporting customization

[KFearsoff]

Currently, the issue-reporting functionality is pretty barebones. It only allows using the generic template and adding more table rows to the metadata table. There are a few reasons to allow more customization: personal preference, adding more blocks, conforming to existing repo issue templates, supporting other forges/bugtrackers.

My use case for this is: I want to add a section to the issue report called "Additional Information" that suggests (in a comment) to add the file with TRACE-level logs, that are already written by default to a tmpfile.

I can contribute a patch, but I just want to make sure we're all on the same page on what's the better solution moving forward :)

A few options I see are (non-exclusive):

1. Make struct body public and let users just specify it themselves
2. Add more knobs to configure the structure
3. Add the knobs to tweak the URL building
4. Natively suggest using Markdown for body using types
5. Nuke most of the existing functionality and let users specify issue reporting as custom hook/POST request?