Support `--' to stop parsing options of sbt-native-packager itself.

[tksk]

When I want my command to take options conflict with sbt-native-packager's one. e.g. -d' $ mycommand -- -d will be eaten by mycommand`

[lightbend-cla-validator]

Hi @tksk,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Typesafe Contributors License Agreement:

http://typesafe.com/contribute/cla

[tksk]

I did sign the CLA.

[muuki88]

Thanks for the contribution. LGTM.

To clarify that I understand everything correctly. The -- separates the build-in commands from your special application commands if they collide with the build-in ones. So

# would just the $debug to 1 but not passed to the application
./start-script -d

with your fix

# enabling debug and sending -d as app argument
./start-script -d -- -d

Can you add an output to the usage command to clarify this?

[tksk]

PTAL.
IMHO, you understand everything correctly.

[muuki88]

Perfect :) Thanks a lot for your time