feat: add CSS processing configuration (resolves #3)

[greatislander]

See: https://github.com/fluid-project/eleventy-plugin-fluid/blob/feat/css-processing/README.md#css

[jobara]

It would be good to add tests for the following:

• The deepMerge function
• Passing custom options to the css through eleventy
• Subdirectories of css files.

[greatislander]

It would be good to add tests for the following:

• The deepMerge function
• Passing custom options to the css through eleventy
• Subdirectories of css files.

The second one was already addressed in the existing test (overriding the basePath to ./fixtures/assets/styles). I've added both a partial in its own directory and a standalone CSS file in a nested directory and tested to ensure they are handled properly and they are.