generate-post-path should be sub-directory aware

[justinabrahms]

Compare:

(file-name-sans-extension (file-name-nondirectory "/path/to/my-file.txt"))
returns: my-file

(file-name-sans-extension (file-relative-name "/path/to/my-file.txt" "/path/"))
returns: to/my-file

[bastibe]

Thank you for your pull request.

Could you comment on when this change is important?

[justinabrahms]

Ah, right. Sorry for that.

If I have a blog post in posts/computer-science/foo.org it will compile out to posts/foo.html. With this change, it correctly goes to posts/computer-science/foo.html

[bastibe]

I see! I had never considered that posts might be nested inside subfolders of posts.

Thank you for the contribution! (Feel free to ping me in this issue or otherwise if I don't release an updated version within a few weeks)