flexible tab / space handling

[ColinEberhardt]

I've just realised that tabs are inserted as ... well ... tabs (yuck). I'd love to be able to configure whitespace handling, or at the very least have the option for visible whitespace so that I know when I've mixed tabs and spaces in the same file.

[micahstubbs]

I like it. the text editor Sublime Text 3 has similar options for configuring how the editor handles whitespace:

[enjalot]

CodeMirror can be customized to support spaces, but it takes at least a little code: codemirror/codemirror5#988

I'm not likely to add this myself, but will review a PR that attempts to.