Added functionality for `article` search without ID specifier.
Added functionality for `article` search without ID specifier.

Helps newbies who use this as a starting point for GraphQL see a typical schema that includes the ability to query by ID of an object, but not the stipulation you must always use an ID. Also changed return of `article` to list per GraphQL convention.

joshprice/hello_graphql_phoenix,joshprice/hello_graphql_phoenix