Allow packaging for different OS than host

[jdimeo]

Instead of automatically packaging for the host OS, I would like to be able to specify which OS to build for. I've used a Windows host to build both a .exe and a Mac .app bundle. You can build more than one package with multiple executions of the plugin.

I've committed some code to do this on my fork:
https://github.com/ElderResearch/JavaPackager/commit/96bf664ba0764c0161dd9a280d0a16a1e1d3cbe0

This introduces a new platform configuration, one of:
auto (the default- current behavior)
mac
linux
windows

Let me know if you want me to open a PR

[fvarrui]

Hi,
I like how you refactored Mojo's execute method. It's cleaner than before.

The generation of the appropriate installer for each platform depends on the platform where JavaPackager is running, so it will be necessary to verify that if the target platform is different than current platform, the installer cannot be generated.

Thanks!

[fvarrui]

And I’ve just realized that a customized JRE couldn’t be bundled

[fvarrui]

Fixed in v0.8.8.