Hyperlink inline images

I'd like to add support for hyperlinking inline images. It appears this isn't supported out of the box, and when I try to create some custom code that wraps the `` tag in an anchor tag, Trix will strip it out.

Strangely, when attempting to link an image on https://trix-editor.org/ via a keyboard shortcut, the box just shows `false` no matter what. This seems like a bug to me.

Additionally, if you open the hyperlink input and immediately click "Unlink", the hyperlink button in the toolbar will become active.

##### Steps to Reproduce

1. Add an inline image
2. Hit ctrl/cmd K to add a hyperlink
3. Observe that "false" is shown in the text field and no link can be saved

##### Details

* Trix version: 1.3.1
* Browser name and version: Any browser
* Operating system: Any OS