Make KSS an optional dependency

[adidahiya]

I want to use documentalist on a TypeScript-only project. I don't want to download kss and its dependencies.

[giladgray]

@adidahiya seems reasonable 👍 . i'd argue we should do the same for typescript. the only plugin that's really required is markdown.

[suchanlee]

bump on this issue. Our project uses node 12 and the latest version of kss currently brings in an older version of nunjucks which requires node to be of version >= 6.9.0 <= 11.1.0.

kss updated its dependency to bringing the latest version of nunjucks which removes the version upperbound but hasn't yet cut a release

[adidahiya]

@suchanlee thanks for the heads up... after looking at optionalDependencies some more, I don't think this is actually what we want for @documentalist/compiler. I've never seen this type of dependency used seamlessly in practice... I think a more appropriate solution would be to split the plugins out into different packages (like @documentalist/plugin-kss) and use regular dependencies. However, this would be a breaking change. Since Node 12 is not an urgent priority for us (until it enters active LTS on 2019-10-22), I would advise staying on Node 10 for now and waiting for kss to cut a release.