Add additional command line parameter for plugins

[Ekopalypse]

Fyi, I’ve created a feature request.
Discussions should take place on GitHub, if necessary.

[Vitalii Dovgan]

There is one more thing that requires an explicit mentioning in the documentation:

• When exactly NPPN_CMDLINEPLUGINMSG is sent to plugins?
• In particular, is it sent before or after NPPN_READY? (Currently it is sent before NPPN_READY. But will it be so in the future?)