Improved EXCLUDED_FILES and PAGES_INCLUDE_POSTS support and configuration

## Improved EXCLUDED_FILES and PAGES_INCLUDE_POSTS

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. 

## Improved Configuration

In #98acb5d I have moved all of the configuration settings out into the main Jekyll configuration file. This makes it possible to share information between this and other plugins, stops data duplication (Why require the user to set the site URL as a variable in the plugin when it should already be set the in configuration file?) and makes it easy to issue future updates without users accidentally overwriting all of their configuration settings, as these are outside of the plugin source.