Add support for width overrides

GDS in govuk-frontend provides the ability to [set explicit widths on elements](https://design-system.service.gov.uk/styles/spacing/#width-override-classes). Their example should form input elements of specific widths, and the use of width override classes is advices on the [table component documentation](https://design-system.service.gov.uk/components/table/) to set table column widths.

We do not currently provide a mechanism to support width overrides.

It's suggested that width overrides could be supported in a similar manner to the `withWhiteSpace` styling function.