Extra headers with halloheadings plugin

[Adam Bowles]

According to https://www.npmjs.com/package/hallo#plugins : You can pass a headings option key "headers" with an array of header sizes (i.e. headers: [1,2,5,6])

However, I can't seem to get this to work

This is the code I'm using:
plugins: {'halloheadings': {headers: [3,4,5]}}