Implement new Sudoku string definition from SudokuWiki.org

[grantm]

See: https://www.sudokuwiki.org/Sudoku_String_Definitions

When a user selects "Open in SudokuWiki.org solver" and ticks the box for "Include digits you have entered" this would allow the pencilmarks to be carried across as well.

Additionally if the app recognised this puzzle string format then we could add a feature allowing people to bookmark a puzzle at a specific point along the solution path (e.g.: the point where they got stuck).