Only compare if filenames are identical

[dingens]

(Having seen your opinion on #58, but I think this is a little different so I ask anyway. Feel free to decline, of course!)

I sometimes have the use case that if files are duplicates, then they also have the same name. In that case, the comparison phase could be sped up a lot because no hashing is needed.

I suggest an option --same-filename or so that causes fdupes to only do a detailed comparison (hashing, byte-wise comparison) if filenames are equal.

Going further, one could also think of an option --same-fullpath: only check files if their full paths (relative to the directory given on command line) are equal.