Add support for Magna-Charta graphs in HTML Publications

Barcharts are added by putting {{barchart}} underneath a table.
Options such as {{barchart stacked}} {{barchart compact}} are possible.

See:
https://github.com/alphagov/magna-charta

Satisfies:
https://www.pivotaltracker.com/story/show/48461623
https://www.pivotaltracker.com/story/show/54702184
Added unit test for correct behaviour of {{barchart}} and {{barchart stacked compact negative}}