Minimal extension: displays indent guides in the text editor for every language.

Features:
- handles mixed tabs and spaces
- handles indent sizes other than four (as well as four)
- uses Visible Whitespace color for guides
- displays guides at the end of whitespace only lines

Fixed since v3:
- doesn't try to put marker at EOF

Known issues:
- no option to show/hide (at all, let alone per-language)
- does not fill in empty lines based on the line above, unless it contains whitespace