Added support for shortening of URLs with directories

In this release, a custom 'cut directories' facility has been added, according to the value of a new constant, mss_cut_dirs. When set, MakeStaticSite removes nesting of folders for URLs that have paths, moving the path's index page to the root. However, it does not truncate directory paths for supporting assets, unlike Wget --cut-dirs, which can be used as an alternative.