Support for content property to change text

[charliecm]

I think it would be useful if we can use the content property to update the content of a text layer. Even though this property is only valid in a :before element, I think in the context of Sketch, it can allow for more applications. For example, when creating a colour palette, we can define the name of the colour and its hex code, which will update the text layers in Sketch:

.color {
    .preview {
        background-color: #00ff00;
    }
    .name {
        content: 'Green';
    }
    .hex {
        content: '#00ff00';
    }
}