Added
* Testers can now specify [external requirements](https://github.com/qtc-de/tricot/tree/develop/docs#external-requirements)
* Requirements are checked before the tester is run and can contain:
* Required files (including checksums verification)
* Required operating system commands
* Required tricot version
Changed
* Improved the selective test functionalities (`--skip-until`, `--continue-from`, `--ids`, `--groups`, `--exclude`, `--exclude-groups`).
Skipped tests are now skipped completely and they neither appear in the output nor start their associated containers.
* Fix bug related to KeyboardInterrupts
* Update test cases to make them more compatible among different distributions