Add StackOverflow social (#243)

Jesus Galvan

10.10.2020 505b2559238c84d57cd82f68f28cce426df2f992

 stackbit.yaml

@@ -93,6 +93,9 @@
            name: facebook
            label: Facebook
          - type: string
            name: stackoverflow
            label: StackOverflow
          - type: string
            name: twitter
            label: Twitter
          - type: string