Props constructor for watchShallow

[glorat]

In the props

vue-plotly/src/Plotly.vue

Line 47 in 3f05b63

watchShallow: false,

Should be

            watchShallow: {
              type:Boolean,
              default: function() {
                return false
              }
            },

because props references types, not values. eslint picks up this bug

Happy to provide a PR for this if needed