-y:
  * soc implemented it via calling sdiff directly, but some options are
  incompatible so it is fragile
  * just recommend the user to run sdiff directly and do not implement it
  * make a libsdiff and use that directly to avoid duplicating the code

to be implemented:
--suppress-common-lines: depends on -y
--ignore-blank-lines
--horizon-lines
--ignore-tab-expansion
--line-format

Will probably be not implemented:
--GTYPE-group-format
--LTYPE-line-format
--speed-large-file: (Do we need that? only a stub?)
--help (We have a manpage already)