WCAG compliance: feather.replace() does not allow adding desc blocks to svgs

In order to conform to WCAG 2.1, SVGs must be written as:
```
<svg>
  <desc>example description</desc>
</svg>
```
or, if the SVG is purely decorative:
```
<svg aria-hidden="true">
</svg>
```
https://www.w3.org/wiki/SVG_Accessibility/Authoring#Nested_decription

Currently, using feather.replace() on a document does not have any way to add those elements to the resulting icons.