Support for content property to change text

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:

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