add flag and functionanlity to URL decode params before comparison and printing.
use constants for url escape char and escape sequence length
updated README with new help message.
added explanatory comment for URL_ESCAPE_SEQ_LEN
make right URL (second positional arg) optional. now url_diff can be used to enumerate a single URL which I think is more readable.
new todo
Merge branch 'master' into avoid_diff_if_equal
small formatting changes of usage output.
Merge branch 'url_decode_before_compare'