Add endPosition parameter to findNextMatchSync.

[Thom1729]

For #79.

This is my first PR for the project, and I don't write much C, so this is probably overly conservative. Input would be appreciated.

• This PR only adds the parameter to findNextMatchSync, not to findNextMatch, search, or searchSync. Presumably we would want to add endPosition to all of the above.
• For clarity, it might be advisable to rename some of the position variables to startPosition.
• -1 is passed to the C code as a null value for endPosition. We could pass the (JS) string length instead, if that makes more sense.
• Docs and tests could be improved.

[rsese]

Thanks for the contribution! As we read this this pull request and the associated issue, it does look like there are benefits for certain use cases, but we don't believe it's something we specifically need for Atom right now and it's not something we would maintain moving forward. For these reasons we've decided not to accept this change and would suggest forking the repository for your particular needs.

Thank you again for your interest and the contribution.