Added strikethrough support to Markdown+Links syntax and Dracula

For a long time, I was using GitHub Markdown as my default syntax but was frustrated at having to switch to link mode. Along game Markdown+Links, which solved that problem but didn’t support strikethrough markdown…

I never gave up on my dream to one day get strikethrough syntax working… and now I’ve done it. Behold, extended syntax and theme with strikethrough support