Comply fully with gov.uk style guide

[ivyleavedtoadflax]

See:

http://govuk-elements.herokuapp.com/
http://style.ons.gov.uk/category/data-visualisation/

Originally #4

[pstils]

https://gss.civilservice.gov.uk/policy-store/introduction-to-data-visualisation/

Shades of blue seem to be the consensus re colours. It would be good if this could be updated here as so many departments will end-up separately developing their own packages to conform with the guidelines.

[ivyleavedtoadflax]

Feel free to open a pull request @pstils 👍