register custom handlebar helper in metalsmith - handlebars.js

Question

I am using Metalsmith to generate a static site from a markdown file.
The people editing the markdown file will write: {{{link "docs/file.docs"}}} and they expect a link on the site with that file (relative link)
The helper is simple, I tested and it is working:
handlebars.registerHelper('link', function(path) {
var url = handlebars.escapeExpression(path);
return new handlebars.SafeString(
"<a href='" + url + "'>" + url + "</a>"
);
});
But how can I add this helper and use it in my metalsmith configuration?
Here is a summarised example.
index.md:
etc etc link to the page is {{{link "docs/file.doc"}}}
I want with a simple make the following part of the html to be created:
etc etc link to the page is <a href='docs/file.doc'>docs/file.doc</a>

Answer 1

I found the answer here: https://segment.com/blog/building-technical-documentation-with-metalsmith/
Here is my index.js config for Metalsmith:
var Metalsmith = require('metalsmith');
var markdown = require('metalsmith-markdown');
var permalinks = require('metalsmith-permalinks');
var handlebars = require('handlebars');
var inplace = require('metalsmith-in-place');
handlebars.registerHelper('link', function(path) {
var url = handlebars.escapeExpression(path);
return new handlebars.SafeString(
"<a href='" + url + "'>" + url + "</a>"
);
});
Metalsmith(__dirname)
.metadata({ title: "Static Site" })
.source('./src')
.destination('/var/www')
.use(inplace({ engine: 'handlebars', pattern: '**/*.md' }))
.use(markdown())
.build(function(err, files) {
if (err) { throw err; }
});