-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:
--horizon-lines
--ignore-tab-expansion
--line-format

Will probably be not implemented:
--GTYPE-group-format (partially implement - minimal)
--LTYPE-line-format
--help (We have a manpage already)
--suppress-common-lines: depends on -y (won't be implemented, as it conflicts
the way sdiff expects it and in any case we have sdiff for that feature)