Should the values for ChronoUnit be kebab case?

[mdedetrich]

After discussion at #331 for adding ChronoUnit to pureconfig, a good point was made in that rather then the appropriate values for ChronoUnit being snake case they should instead be kebab case (or maybe something else more appropriate). The value that this effects is

HALF_DAYS("HalfDays", Duration.ofSeconds(43200))

Currently with my PR at #331 we accept either HALF_DAYS or half_days but maybe another format such as half-days is more appropriate.

There is also a discussion whether rather than accepting both HALF_DAYS and half_days that instead we should only accept a single standard format.

[bkirwi]

I don't have strong feelings on this one... leaning towards being as generous as possible in what we accept, since there's no clear winner, but open to the other approaches as well.

[mdedetrich]

Will close this for now, if we decide to do something more fancy like whats done with pureconf a separate generic issue should be made about it