Update ingredient spec to allow ingredients to export metadata

[lzilioli]

This issue maps back to ROUX-530 in our private issue tracker.

Ingredients may want to specify tags or other properties that the rest of the roux tooling can use. For example, roux-webpack-config may want to automatically generate an entry map of ingredients, but can't do so unless the ingredients have some way of indicating that they should be treated as a webpack entrypoint.

• update the ingredient spec to allow an ingredient.json or yaml-front-matter in ingredient.md
• upgrade the roux code to read this metadata and include it in the ingredient objects returned from roux.resolve