Patch for Typo migrator to support extended posts and posts without slugs

I just migrated my Typo blog using this, and ran into some issues, because I had a lot of posts with the "read more" option.  I tweaked this migrator a bit to query the 'extended' column as well, and put the necessary <!-- more --> tag into it for jekyll and append the extended content into the main post.

I also had a couple of blog entries that had no slug on them somehow - and that caused the migrator to hang.  I added a check for this being nil, which replaces it with 'no slug' so it can keep processing if that's encountered.