Column-gap + Fix for Nim 1.6

The CI now checks correctly that the code works on the latest Nim 1.6.x. One bug was found and fixed.

The gap between columns can now changed by passing the width of the gap in em:

let gap = 2.0
columns(gap):
  column:
    nbText: "Hi"
  column:
    nbText: "Bye"

What's Changed

• add column-gap parameter by @quimt in #36
• use correct input argument in CI by @HugoGranstrom in #47

New Contributors

• @quimt made their first contribution in #36

Full Changelog: v0.2.5...v0.2.6