`mj-include` unable to load included template when no unique root element

When the included file doesn't have a unique root element, the parser fails with the "unable to load included template" error message:

```mjml
<mj-button> Test </mj-button>
<mj-button> Test </mj-button>
```

It seems like a root node is needed:

```mjml
<mj-column>
  <mj-button> Test </mj-button>
  <mj-button> Test </mj-button>
</mj-column>
```