Support specifying user preferences in the CLI

[kasperisager]

With #259, user preferences are supported as device parameters. However, there's not yet a way of specifying user preferences when working with the CLI. This could be done using a --preference flag that works similarly to the --header and --cookie flags. As such, one could do the following to audit a page with reduced motion enabled:

$ alfa audit --preference prefers-reduced-motion:reduce <url>

Another option is having each feature be a flag of its own:

$ alfa audit --prefers-reduced-motion <url>

This could lead to a lot of additional flags being added so I'm learning more towards the first approach.

[Jym77]

I'm also in favour of the first approach.

[kasperisager]

The nail in the coffin of the second approach is also that it couples the CLI to the available user preferences, which would not be the case with the first approach.

[kasperisager]

We'll go with the first approach so this should mostly be a matter of duplicating the existing functionality for headers and cookies.