Make `--strict` mode default

[mpkorstanje]

Depending on the outcome of the discussion in cucumber/common#714 we should make --strict mode default and possibly remove it all together from Cucumber.

This will make integration with other frameworks and reporting tools easier as they will only need to understand the standard variations of passed, skipped and failed.

Deprecation strategy:

• v5.x.x: Print out a warning when tests are not ran in strict mode [Core] Print warning when using --non-strict #1835
• v6.0.0: Remove snippet from summary printer (is included in exception - except for CLI) [JUnit/TestNG] Always print snippets and summary #1790
• v6.0.0: Default to strict and ignore strict mode input --strict option, error on --no-strict.
• v7.0.0: Remove --strict and --no-strict options.