Update _config.yml

added prose.io config 

 - name: "title"
        field:
          label: "Title"
          element: "text"
          value: ""
      - name: "tags"
        field:
          element: "multiselect"
          label: "Add Tags"
          placeholder: "Choose Tags"
          options:
            - name: "Ideas"
              value: "ideas"
            - name: "Startup"
              value: "startup"
            - name: "Tech"
              value: "tech"
            - name: "Life"
              value: "life"
            - name: "Productivity"
              value: "productivity"