Markdown: strikethrough does not work consistently

#### Steps to reproduce
1. Add a note on the MacOS Simplenote app
2. Use the markdown for strike through ~~like this~~ (~~ on either side)
3. Preview the markdown and/or publish the note

#### What I expected
To see the strikethrough, as it works on the web app and Android previews

#### What happened instead
Did not format the strikethrough properly

Note: This also does not work on the Published note, from any Simplenote app.

#### Simplenote version
1.3.9

#### macOS version
10.14.1

Reproduce the issue in the Simplenote macOS app using the listed strikethrough steps, then compare preview and published-note behavior with the web and Android previews. Done means ~~text~~ renders consistently in both the macOS preview and published note.

Written by the indexing model from the issue text.