Cmd+K to create links

### What
When a text is highlighted, hitting Cmd+K will transform it to a link to the URL in the "copy" memory (i.e., the link that would be pasted should we press on Cmd+V).

### Why
Links in Markdown can be a bit cumbersome. I use simplenote to store many links and this feature would make simplenote much more productive for me. Besides, Cmd+K is a well-known shortcut for creating links in Markdown (e.g., [Typora](https://support.typora.io/Shortcut-Keys/#format), or even GitHub). Greater compatibility between Markdown editors is always a plus.

### How

The issue names no files or tests; begin by locating the macOS Markdown editor and the existing Cmd+V clipboard handling. Verify the highlighted text becomes a Markdown link using the URL currently in the copy memory, and confirm the behavior with the illustrated Cmd+K workflow.

Written by the indexing model from the issue text.