Improved EXCLUDED_FILES and PAGES_INCLUDE_POSTS support

Changed the `EXCLUDED_FILES` and `PAGES_INCLUDE_POSTS` arrays to support the relative paths of files. This makes both features much more useful for those who structure their sites by making use of folders with index.md/html files in in order to keep URLs tidy. 

So, if you now want to target just the index.md file in the notebook folder you could use `/notebook/index.md` which will target just this one file, whereas previously you would have had to use `index.md` alone which would have included every file called `index.md` rather than just the one. 

Essentially, this makes the `EXCLUDED_FILES` and `PAGES_INCLUDE_POSTS` arrays more useful in a much larger range of situations.