Add builder derive and non_exhaustive for option structs

[YJDoc2]

This implements the suggestions from #174 (comment)

This adds builder for each of the option structs and a non_exhaustive annotations on all of them.

It goes without saying that this is a breaking change due to the non_exhaustive.

[YJDoc2]

@kivikakk , I have added the builders, however the usage seems a bit too verbose to me.
Do you think flattening all the options into just ComrakOptions might be a better idea? That means we would not have separate parse,render and extension options.

[kivikakk]

Hmmm, yep, I totally get what you mean -- it does seem pretty verbose. I think it does make sense to remove the distinction between the three different structs when using builders; that was essentially to make it easier to provide a smaller set of options, which builders help with.

That said, I'm a bit worried that it'll be less obvious what distinct kinds of options are available. This definitely ties into #291 in a big way, so I'm not exactly sure yet which direction to go.

Thanks so much for this -- I'm going to think on it now that I can really see what it looks like. I think it's close but not quite it.

[kivikakk]

Thanks for your work here, I really appreciate it! I've rebased it after this came up again in #303, #305 and #321 (!). One thing I left out from that (and this) PR is the root options struct. I'm still not convinced that we shouldn't just merge all the options into one big struct, but it still seems too ugly -_-

[YJDoc2]

Glad to help!

[Turbo87]

error[E0433]: failed to resolve: could not find `OptionsBuilder` in `comrak`
  --> crates_io_markdown/lib.rs:64:17
   |
64 |         comrak::OptionsBuilder::default();
   |                 ^^^^^^^^^^^^^^ could not find `OptionsBuilder` in `comrak`

maybe I'm using it wrong, but it looks like those builder structs are not pub. maybe introduce a fn builder(Self) on the options structs?