provide JGivenStage outside of spring context - for kotlin allopen

[jangalinski]

When using kotlin, all stage classes and functions have to declared open to get JGiven working.

The kotlin compiler plugin "all-open" can do this for you, which is very convenient. But it requires an annotation on class level to work.

The JGivenStage annotation used in jgiven-spring is a solution for this, but what if you do not use spring?

I ended up defining a custom JGivenStage annotation twice in my kotlin projects, so I would like to have a "centralized" solution.

Maybe introducing a jgiven-kotlin module could be a good place for this?

Would you be open for a PR?

[janschaefer]

I am always open for PR :-). The question is whether a whole jgiven-kotlin module is not a bit of an overkill for just a single annotation. How would such an annotation have to look like?

[jangalinski]

see #407