Add a <spoiler> & <detail> options to editor/notes Merged

Often I write notes which are relevant to multiple audiences. Having a <spoiler> / <detail> tag would let me specify more technical content and make it opt-in for the reader. That way I could include more technical content but avoid overloading less that-kind-of-technical readership who would also benefit from same info.

Ideally I could lay out notes with various sections like:

Description of [part of system].

<spoiler>
  <detail>Code walkthrough for [part of system]</detail>
  lotsa code and other goodies in here which aren't relevant to all audiences!
</spoiler>

then there would be a little < v caret next to "Code walkthrough for [part of system]" readers could click on to open the semi-relevant portions.