Feature request: use <ins> and <del> for Diff output

[Jed Laundry]

Is your feature request related to a problem? Please describe.
The output from the Diff operation uses span classes which colour the output, but this output can be difficult to copy/paste into other applications/documents.

Describe the solution you'd like
Instead of using <span class='hl3'> and hl5, use <del> and <ins>, which are rendered with strikethrough and underline.

Describe alternatives you've considered

• We could output the diff with Markdown, but Markdown doesn't have a defined underline
• We could provide a dropdown for how the Diff output should be rendered, but a dropdown with 2-3 items seems overkill