Add option to display invisible/unprintable characters #2150

This is already mentioned as "Possible Future Additions" in #2073, but I thought it would be helpful to have a dedicated issue to track the implementation.

Ideally, it should be possible to visualize non-printable characters, control characters, zero-width spaces, etc. both when viewing the source code of a file, in code blocks in comments, and on diffs. Especially on diffs.

See for example how Diffchecker does it (note the red dot after "Lorem" on the left text box):

<img width="1125" alt="Screenshot 2019-06-11 at 17 52 42" src="https://user-images.githubusercontent.com/478237/59291013-cffa6f00-8c71-11e9-8a95-99ff974768d2.png">