main and ignore are optional, but bower complains if missing

[huerlisi]

Since bower/bower#694 has been fixed, bower warns the user when installing modules whose bower.json does not include main and ignore.

But the spec still specifies them as optional. I think these attributes should be marked as required in the spec.

[Daniel-Hug]

+1. The docs page implies that they're optional: http://bower.io/docs/creating-packages/#bowerjson