Share By Email Extension: Prefill recipient's address

It'd be nice to be able to provide an optional default recipient address for the *Share By Email* extension.

Currently the URLs are formed like this:

```
mailto:?subject=foobar&body=https://blog.example.org/post-1
```

With a default recipient address (that has to be entered as option for *Share By Email*), the URL would look like:

```
mailto:defaultrecipient@example.org?subject=foobar&body=https://blog.example.org/post-1
```