Standalone SPI registration

[tvasenin]

Currently, If I need to add JPEG2000 support to the application, I have to deal with all other plugins brought automatically by jai-imageio-core. This is pretty annoying, as it's often not possible to correctly unregister these plugins, especially when your project is a library and not an application.

It makes sense to be able to add JPEG2000 plugin independently from the remaining core plugins, especially given there's support for those formats in Java9+ and other actively maintained projects, like TwelveMonkeys.