[Feature] Add optional divisor to convolute()

**Is your feature request related to a problem? Please describe.**
When searching the Internet for convolution kernels, one often finds some which normalize at the end of the calculation with division.

A typical example for a Gaussian blur might look like this:

```
| 1 4 6 4 1 |
| 4 16 24 16 4 |
| 6 24 36 24 6 |
| 4 16 24 16 4 |
| 1 4 6 4 1 | / 256
```

To find the equivalent version which doesn't include a divisor step, one would need to laboriously divide each of those integers with the divisor, resulting in something which might look like this:

```
[0.00390625, 0.015625, 0.0234375, 0.015625, 0.00390625],
[0.015625, 0.0625, 0.09375, 0.0625, 0.015625],
[0.0234375, 0.09375, 0.140625, 0.09375, 0.0234375],
[0.015625, 0.0625, 0.09375, 0.0625, 0.015625],
[0.00390625, 0.015625, 0.0234375, 0.015625, 0.00390625]
```

**Describe the solution you'd like**
What would be nice would be if the `convolute()` functions added another (optional) argument called `divisor` which defaults to `1`.

**Describe alternatives you've considered**
Noting that my app is a Node.js daemon, I was able to monkey-patch `node_modules/@jimp/plugin-color/dist/index.js` inside the `convolute()` function definition like this:
```
var ksize = (kernel.length - 1) / 2;
x = isDef(x) ? x : ksize;
y = isDef(y) ? y : ksize;
w = isDef(w) ? w : this.bitmap.width - x;
h = isDef(h) ? h : this.bitmap.height - y;
var source = this.cloneQuiet();
// Begin edit ------------------
var divisor = 256; // Imagine instead that the function accepted it as an argument
// End edit --------------------
this.scanQuiet(x, y, w, h, function (xx, yx, idx) {
var value = applyKernel(source, kernel, xx, yx);
// Begin edit ------------------
value[0] /= divisor;
value[1] /= divisor;
value[2] /= divisor;
// End edit --------------------
this.bitmap.data[idx] = this.constructor.limit255(value[0]);
this.bitmap.data[idx + 1] = this.constructor.limit255(value[1]);
this.bitmap.data[idx + 2] = this.constructor.limit255(value[2]);
});
```

It worked perfectly, normalizing the darkness/lightness of the resulting blur. Without this, it's problematic coming up with a good-looking matrix whose values sum to one (unless you have access to Matlab).

Of course, there appear to be several interfaces other than just `@jimp/plugin-color/dist` but I didn't use those.

Start with the convolute() implementation in @jimp/plugin-color/dist/index.js and trace the other convolute interfaces mentioned in the issue. Add an optional divisor that defaults to 1 and applies to the calculated color-channel values before limiting them to 255. Done means normalized kernels such as the provided Gaussian example produce the expected brightness.

Written by the indexing model from the issue text.