Allow additional label classes in radios and checkboxes macros

[kr8n3r]

At the moment the label classes on radio and checkbox items are fixed. There has been interest to be able to modify them, to say make the label bold.

When combined with ability to pass through then hint, the macros will be much more customisable

This PR:

• Updates the component template
• Adds a test to check for the additional label class
• Updates READMEs

Screenshot

[joelanman]

I think we might need a quick discussion about whether bold radio labels is something with a definite need. For example you could differentiate with normal labels, and grey hint text.

jani added: as done here https://reply-jury-summons-trial.service.gov.uk/steps/00-responder-type

[kr8n3r]

Prior art:
https://www.gov.uk/log-in-file-self-assessment-tax-return/sign-in/prove-identity

https://govuk-frontend-review.herokuapp.com/components/radios/with-a-medium-legend/preview

https://claim-power-of-attorney-refund.service.gov.uk/application/who-is-applying

https://get-state-pension.service.gov.uk/confirm-identity

https://appeal-tax-tribunal.service.gov.uk/steps/appeal/case_type

[joelanman]

the second example is interesting because it puts the hint inside the label. I think that just highlights that we should probably discuss what the best way to use radio labels is

[kr8n3r]

sure, it puts in there as it was (is) the only way to do it.
but yeah, discuss

[NickColley]

We used the bold to contrast with the hints better I think.

[kr8n3r]

we have decided to go ahead with this. code review needed

[36degrees]

LGTM. Thanks @igloosi!