Make all config options optional

[dcastro]

Clarification and motivation

In #126 we made the ignoreRefs option required (to match the other options).

However, in a subsequent discussion, it was pointed out that having it optional is better for backwards-compatibility and to help users migrate to newer xrefcheck versions.

Let's make all the config options optional.

Acceptance criteria

• All config options are optional

[dcastro]

Now that this is merged, I think we should go over the multiple test config files in tests/golden, and trim them so that only the relevant options are present (see example: #152 (comment)). @Sereja313 can you please open another PR linking again to this issue?