I'm looking for advice on whether this belongs...
... in core, and I'll write a PR, or...
... in a plugin, and I'll write the plugin (something I've already done).

I have a project with many top-level files and directories, including (perhaps obviously) node_modules. Of those, I want to use two as the source for Metalsmith: posts/ and index.html. I could use . as the source and manually ignore the rest (especially node_modules, which breaks everything), or (as I've done at the moment) I can use posts as the source, authoring a simple plugin to manually .read() the index, adding it to the files Object, etc.

Which of the two options is best? Am I missing a third, even better option?