Handlebars statements in code blocks are removed in custom helpers are included

[pzuraq]

If options.helpers is set and custom Handlebars helpers are included, the second pass on the markup of examples with Handlebars removes all unregistered helpers entirely. In the case where some examples have been included in a code block in Markdown, like below, this will result in an empty code block.

/**
 * ```hbs
 * {{#test-component}}
 *   Some text
 * {{/test-component}}
 * ```
 */