Increase gap between columns to ensure that left rounded borders of highlighted boxes in the right column are not overlaid by the left column

Style sheets can now be added in CSS or SCSS format. SCSS files are
automatically compiled to CSS and added as a dependency to whatever
target uses them. Which means changes to SCSS files are picked up by
`decker watch` and `decker server`.

- No more abuse of TemplateHaskell to embed files in the executable
- Speeds up compilation and linking considerably
- Uses external programs zip`, `unzip` and `cp` though
- May not work on windows