Replace outdated parts of plugman with cordova-plugin-template and documentation

[janpio]

Those can probably be removed:

Author plugins
    create               Create a plugin
    createpackagejson    Add a package.json file to a plugin
    platform             Add and remove platforms from a plugin

Some input:

• Whatever plugman create, plugman createpackagejson and plugman platform create.
• https://kerrishotts.github.io/pgday/workshops/2017/campp/
• https://github.com/kerrishotts/cordova-plugin-example-isprime
• https://github.com/don/cordova-plugin-hello

Oh look, something like this already exists:
https://github.com/phonegap/phonegap-plugin-template
https://github.com/cesilva/cordova-plugin-template

Unfortunately this is gone:
https://www.npmjs.com/package/cordova-plugin-template
Although we probably don't need to publish this to npm anyway.

[janpio]

TODO: Check if the actual library code here is used in Cordova somewhere else.

[raphinesse]

TODO: Check if the actual library code here is used in Cordova somewhere else.

This repository contains only the CLI of plugman. All actual functionality is implemented in cordova-lib/src/plugman. This "plugman library" contains some parts that are not used by anything but the plugman CLI and some parts that are also used during certain operations of Cordova CLI. I'll gladly do the deed of removing all unnecessary cruft, should we decide to do this.

Cordova package dependency graph for reference:

[janpio]

TODO done then - not necessary to think about that ;)

[janpio]

Some more reference material:

• https://github.com/MicrosoftDocs/cordova-docs/blob/master/articles/develop-apps/create-plugin-windows-phone.md
• https://github.com/purplecabbage/cordova-runtimecomp-echoplug