Improve resistor-color-duo: handle solutions based on reduce

Currently, when a solution contains `reduce`, we _explicitly_ bail out early. The following two examples should be considered _approvable_, but **not** optimal. Both should come with a hint.

```
const value = (colors) => {
return Number(
colors
.slice(0,2)
.reduce((digits, color) => digits + colorCode(color), '')
)
}
```
Use the type-juggling message hint.

```
function value(colors) {
return colors
.slice(0,2)
.reduce((value, color, index) => value + colorCode(color) * Math.pow(10, 1 - index), 0)
}
```
Use a new hint: Is `reduce` really the best candidate when trying to get the sum of significant digit values, when there are _two_ values only.

**Which exercise**

`resistor-color-duo`