Define scala versions keys (eg scala212.value)

[dwijnand]

Fixes #11

WDYT @djspiewak?

[djspiewak]

What about making it Option[String], so we can be a bit more typeful about the ill-defined scenarios? I'm in favor of keeping the warning, just thinking about the exact value.

[dwijnand]

It's the more obvious type, but ergonomically annoying in 99% of cases. Thus the sanity check opt-in instead.

[dwijnand]

I'm not going to release this quite yet (I don't need it quite yet), so I'll merge this as is and we can forward-fix (or even revert - no sweat) this before releasing.