s-match: Save match-data and support START parameter.

`s-match' should probably save external match-data or the behavior
should be documented.  Support for the START parameter of
`string-match' was also added.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Add START parameter to s-matches?.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Merge pull request #21 from ruediger/s-match

s-match: START parameter