add kronk prune command to delete old run history

Merge pull request #9 from janpgu/add-db-tests

add tests for db and worker packages, fix NULL stdout/stderr scan bug

update contributing section in README

improve README wording

handle strconv.Atoi errors in parseHourMin

Merge pull request #8 from janpgu/fix-double-wrapped-errors

fix double-wrapped error messages in edit command

Merge pull request #7 from janpgu/fix-repeated-cron-parsing

fix repeated cron parsing in edit command

Merge pull request #3 from janpgu/add-kronk-version

add kronk version to README commands table

add kronk version command

Merge pull request #2 from janpgu/add-pause-resume

add gitattributes for consistent line endings