Add option to create reference links when pasting links into markdown

1. Copy `http://example.com`
2. Open markdown editor and paste

**Current behavior**
Pastes with option to insert md link: `[text](http://example.com)`

**Feature request**
Also give option to add reference link instead:

```md
[text][ref]

[ref]: http://example.com
```

Presumably should reuse existing references too if one already exists