Optionally allow missing lists in yaml/json-to-dhall

[akshaymankar]

Fixes #1410

[sjakobi]

This looks great! My understanding of #1410 was that we also want the --omissible-lists option to replace --omitEmpty in dhall-to-json and dhall-to-yaml. Would you like to implement this too? Otherwise this could be done in a separate patch.

[akshaymankar]

Sure I can do that also in this PR. As this would be a breaking change, would it be better to have deprecation warnings for omitEmpty?

Also, should I rename --omitNull to --omit-null (omissible-null sounds weird) and --noMaps to --no-maps for consistency of case?

[sjakobi]

As this would be a breaking change, would it be better to have deprecation warnings for omitEmpty?

It might be a good idea to keep --omitEmpty around for a bit. AFAIK we haven't used actual deprecation warnings so far, so I don't think it's necessary to add some in this case.

Also, should I rename --omitNull to --omit-null (omissible-null sounds weird)

--omitNull is the default setting now, so I think we can simply get rid of it soon.

and --noMaps to --no-maps for consistency of case?

Yes, let's get rid of camelCase in CLI options. Not sure whether we need to keep --noMaps around again… Thoughts @Gabriel439?

The new --preserveNull option should also be changed to --preserve-null.

[sjakobi]

Seems like the updating of the CLI options would be a well-sized change on its own, so let's maybe merge this as-is and change the options in a follow-up PR. Would that be ok for you, @akshaymankar?

[akshaymankar]

Yeah that would be ok with me, feel free to merge :)