Fix regex finding post_id

Previous regex matched numbers in blog name (e.g. **3**mangos.tumblr.com) instead of post id.
Better emphasis in comment

plus a link back to github!
Regex fix #2

Literally minutes after the last fix, I ran into an error with it. Hopefully this will work.
Merge pull request #4 from kleptoc/master

Regex fix for tumblr handler