Callback on completion and asterisk output

In this release we see the GT refactoring complete and two new features. The first is a callback function for when a process completes. The second is to remove the word ERROR from STDERR output - this might not be an error - instead, we use an asterisk to mark that the output is going to STDERR.

What's Changed

• feature: callback functions on process completion closes #17 by @g105b in #55
• fix: asterisk for stderr output by @g105b in #59
• Add JetBrains sponsorship to README with H1 heading by @g105b in #53
• Gt->GT tidy by @g105b in #57

Full Changelog: v1.1.5...v1.1.6