npm-use-integrated option to disable using integrate npm and use systemwide installed npm instead

[onsquareDev]

option will invoke systemwide installed npm instead #70
also systemwide npm invokes npm install instead of npm update #51

[mkurz]

Thanks, I will come to this pull request while working towards the next Play Framework major release.

[mkurz]

Starting with version 1.3.0, when using a local installed js engine that supports node, sbt-js-engine tries to detect the systems npm command and will use it instead of the npm webjars.
This is the new default and can be disabled with npmPreferSystemInstalledNpm := false, to keep using the webjar npm, no matter what the engine is.

Also, starting with 1.3.0, it will be possible to set which npm subcommand is used by setting npmSubcommand := NpmSubcommand.Install (available options are Install, Update, Ci). update will stay the default however.