function link (longname, options) {
  return options.fn(_link(longname, options));
}
@param id {string} - the ID to link to, e.g. `external:XMLHttpRequest`, `GlobalClass#propOne`
etc.
@static
@category Block helper: util
@example
{{#link "module:someModule.property"~}}
  {{name}} {{!-- prints 'property' --}}
  {{url}}  {{!-- prints 'module-someModule-property' --}}
{{/link}}