Add support for github wiki pages

The links generated by doctoc in github.com mode are like `[link](#anchor)` but such links don't work when you are using the github wiki repository (not the actual repository, but the link to a wiki on top) and for those cases links should look like `[link](page#anchor)`

This solution might not be the optimal one but that's the best I could do with what I know about nodejs, so you might want to do it your own way (or not add it at all, that's up to you)

PS: Sorry about removing your herokuapp link from the header of the table