feat: support custom split functions in createCommandReaders

[gerrnot]

Default is (still) split by line, but in some cases interesting command output spans multiple lines (e.g. commit messages)

This also provides a split function definition that splits by a string.

• [x] All tests passed. If this feature is not already covered by the tests, I added new tests.
• [x] This pull request is on the dev branch.
• [x] I used gofmt for formatting the code before submitting the pull request.
• [ ] I labeled this pull request with one of the following: 'breaking change', 'new feature', 'bug', or 'ignore for release' ⓘ I would like to flag this as 'new feature' but I seem to lack permission!

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[gerrnot]

If somebody has a hint why frogbot is failing here I would be grateful. There i no output, just says it fails, but why?