adding a new function called expandFilePaths

adding a new function called expandFilePaths in Guard, that will return the complete file paths after filtering out zero byte files. (advantage is, user can have a work around with the file names before load if required) . Filtering out zero byte files can be used to avoid the empty-reduce left exception when there are no files to be processed. This check can be done by checking the size of the list returned by expandFilePaths.
[ User can either make use of the result of expandPaths or the result of expandFilePaths to be passed to the load function.]
It took me a little while to understand (I think I understand!) what this method does. My first impression was that it operates on a list of files (as per the comment: "expand the complete file paths"). It actually operates on a list of directories, and returns the non-empty files contained within. Could we rename the method so that this is immediately obvious?

Maybe `expandDirectories`? `listFiles`? `listNonEmptyFiles`?