Add the ability to specify HTML for a navigation item

[adamsilver]

No description provided.

[36degrees]

@adamsilver Thanks for raising this.

Before this can be merged, we'll need to:

• add html to the params list in /src/govuk/components/header/header.yaml:

  govuk-frontend/src/govuk/components/header/header.yaml

  Lines 22 to 30 in 9a77bee

  	- name: navigation
  	type: array
  	required: false
  	description: An array of navigation item objects.
  	params:
  	- name: text
  	type: string
  	required: false
  	description: Text of the navigation item.

• add a test for this behaviour to /src/govuk/components/header/template.test.js

• document this as an unreleased new feature in /CHANGELOG.md

Are any of those things that you're comfortable doing?