v0.6.1

• @IndrajeetPatil is now a ggsignif author in recognition of his significant
  and sustained contributions.

• Add extend_line parameter to make the brackets shorter or wider (#70).
  Thanks to @romanhaa for making the PR.

• Adds a new package website: https://const-ae.github.io/ggsignif/.

• Minimum ggplot2 version needed is bumped to 3.3.3.

• Minor internal refactoring

• Adds snapshot and visual regression testing infrastructure.