Add option for including release notes in update dialog

Just wondering if showing the release notes when prompting the user to restart is on the roadmap? Release notes would be taken from the github release.

Here's a potential mock-up with a toggle to show/hide:


An option could toggle whether to show them or not:

```js
require('update-electron-app')({
repo: 'ummoji/ummoji-desktop',
releaseNotes: true
})
```

Even if release notes are turned on they would only be shown if there are actually release notes associated with the release. The release notes would probably need to be contained with a maximum height and overflow content should scroll because otherwise the height of the dialog may be too large. Perhaps this should be configurable or perhaps not.