Add an init command

To allow people to use plonecli command like build and serve in existing packages, we could provide an init command which will prepare a directory for plonecli.

What it needs is documented here:
https://bobtemplatesplone.readthedocs.io/en/latest/upgrade-packages.html#bobtemplate-cfg

We basically need a bobtemplate.cfg in the root directory of a package or buildout.
We should ask for a bobtemplate name like addon and write it to the bobtemplate.cfg.
The template name should be a list of existing templates, comming from the plonecli registry.
It has to be the global name, 'plone_addon' not just 'addon'.
Because the configuration is for mrbob and not only for bobtemplates.pone.

The command can be used then like:

plonecli init mybuildout

or just without a directory, for the current directory

plonecli init

also we need to update configure.zcml files to have the placeholder lines, where we inject config.

Start with the upgrade-packages documentation linked in the issue and trace the existing plonecli command entry points and registry handling. Implement plonecli init [directory] so it writes a root bobtemplate.cfg using a selected global template name, and update configure.zcml with the required placeholder lines; verify both current-directory and named-directory usage.