as:

  "whatever", :file_in_same_dir

or

  "whatever"::file_in_same_dir

.__init__(): relax the second gsub, using a '*' instead of a '+', so
the stuff before the ':' can be missing, and also do postprocessing so
any resulting '<a href=":file_in_same_dir">'s have the superfluous ':'
removed.  *Seems* good!