code: function addPartial(filename, done) {
        // Create a copy of the file and delete it from the database.
        const file = clone(files[filename])
        delete files[filename]
  
        // Compile the partial.
        const info = path.parse(filename)
        const transform = info.ext ? info.ext.substring(1) : null
        const transformer = getTransformer(transform)
        if (transformer) {
          // Construct the options.
          const options = extend({}, metalsmith.metadata(), file, {
            filename: path.join(metalsmith.source(), filename)
          })
  
          // Compile the partial.
          transformer.compileAsync(file.contents.toString(), options).then(template => {
            /**
             * Define the partial as a function.
             */
            metalsmith.metadata().partials[info.name] = locals => {
              const opt = extend({}, options, locals)
              return template.fn.apply(file, [opt])
            }
            metalsmith.metadata().partials[info.name].file = file
            done()
          }, done)
        } else {
          // Do not error out when the Transformer is not found.
          done()
        }
      }
docs: Add the given file in as a partial.