Bug: Markdown for horizontal lines changed behavior

#### Steps to reproduce
1. Add --- or ----- to a note that has markdown enabled in the Simplenote Mac app v1.3.8
2. Preview the note in the Mac app

#### What I expected
I expected it to display a horizontal line, which it used to do and still does in the web app.
Screenshot from the web app: https://cloudup.com/cGIcBUYTg99

#### What happened instead
Instead it shows 3 dots in a row for either line (3 hyphens or 5)
Also, the dots are blue and very small, making it difficult to see
Screenshot from the Mac app: https://cloudup.com/cw-nwA3zrRe

#### Simplenote version
Simplenote MacOS 1.3.8

#### macOS version
10.14

Reproduce the issue in the Simplenote Mac app with Markdown enabled using --- and -----, then inspect the Mac app's Markdown preview path and compare it with the web app behavior shown in the screenshots. Done means both inputs render as visible horizontal lines rather than three small blue dots.

Written by the indexing model from the issue text.