Messages: strikethrough text is not parsed

[drygdryg]

For strikeout text in the Rocket.Chat markup, text is framed with single tildes ("~strikeout text~").
By default, Showdown does not parse this. It supports the strikethrough option (disabled by default), but it only applies to double tildes (not single tildes like in Rocket.Chat).
Example message:

Test ~strikethrough text~

After converting to Matrix: