Add URL Scheme Support

### What
The iOS app offers a URL scheme that can be used to create new notes in the application:

simplenote://new?content=encodedContent&tag=encodedTag

When that URL is loaded on iOS, the Simplenote application is opened with the content and tags set within the URL.

The macOS app should have feature parity in this respect with the iOS app.

### Why
This would allow the Simplenote application to be easily automated with tools like Automator for creating notes with specific templates set within the URL.

Start by locating the macOS app's URL-handling entry point and compare its behavior with the iOS URL scheme described in the issue. Done means opening the app from simplenote://new URLs and creating a note with the encoded content and tag values applied.

Written by the indexing model from the issue text.