Better emphasis

[luciansmith]

Description of the change

Don't replace internal underscores with emphasis tags. This was particularly important for URLs, which are often littered with them, and none of them should instead have <em> in them; that's invalid HTML.

The same change should be made for strong tags (like this) but the same fix there broke overall parsing. Stubs provided; maybe someone can figure out what went wrong.

Checklist for contributor

• [ ] if you want to be mentioned in the AUTHORS file, you added yourself
• [ ] added an entry to CHANGELOG.md at "Upcoming"
• [ ] if any Markdown definition changed, you updated the definition docs
• [ ] your C++ code change is accommodated with a unit/integration test where it makes sense
• [ ] your code meets the code format style (clang-format) of the project (tools/format.py)