Add strike-through extension from peg-markdown

[rickharrison]

https://github.com/jgm/peg-markdown now has a strike-through extension. Would it be possible to merge this in to your fork? Any tips on where to start and I can try and tackle this myself.

[dreamwieber]

I would start with a diff of the peg files that are still in this project with the changes in the source repo and then merging those in -- probably by hand, since this repo has diverged a good deal.

You'll have to look at the large switch statement that appends the attributes to the source string, add a case for strike-through, and apply the appropriate nsattributedstring attribute.

Hope this helps!