Props constructor for watchShallow

[glorat]

In the props

https://github.com/statnett/vue-plotly/blob/3f05b63a4953829cbce657be4c36f6ecec178e12/src/Plotly.vue#L47

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