Support additional context for presets

[nhoizey]

For example, the same content with images can be included in the main area of the content's individual page, in a thinner column in another page, etc.

Presets could be specialized with a context:

• preset-name for default behaviour of the image
• preset-name:context-name for specific behaviour of the same image in a specific context

preset-name is attached to the image as usual, but context-name is an additional parameter of the computing function.

With the Eleventy plugin, images-responsiver could be exposed as an Eleventy filter, so that developers can pass the context as additional parameter in different templates.