add plot modification after declaration and initialization

I want to save my plot to a variable, and then add or change some parameters.

For example, I have a plot and I want to use it with different flavours

Right now, my code looks like this:
```kotlin
val myPlot = plot {
line {
x(listOf(1, 2, 3))
y(listOf(5, 3, 1))
}
}

(plot.features as MutableMap)[FeatureName("layout")] = Layout(flavor = Flavor.DARCULA)
plot.save(...)

(plot.features as MutableMap)[FeatureName("layout")] = Layout(flavor = Flavor.HIGH_CONTRAST_LIGHT)
plot.save(...)
```

Start by tracing how the plot builder creates the plot, how `features` stores layout values, and how `plot.save(...)` consumes them. Define what post-initialization changes should be supported and how the same plot can be saved with different flavors; done means the example works without directly casting `features` to `MutableMap`.

Written by the indexing model from the issue text.