internal/mongodoc: add Contents field to mongodoc.Entity

This will allow us to gain quick access to selected files
in an entity's archive such as README and icon.svg.
Merge pull request #184 from rogpeppe/077-zip-contents

internal/mongodoc: add Contents field to mongodoc.Entity

This will allow us to gain quick access to selected files
in an entity's archive such as README and icon.svg.

We'll actually use the field in an upcoming PR.