Here is what I came up with so far for the interaction of os and installer plugins.

- based on what we discussed info for used installers and installer priority
   can come from user config > os plugin > installer plugin
 - the InstallerContext manages the selected os and selects the configured installers.

@wjwwood as basis for discussion later
I decided to go for singleton for `verbose`, since it is kind of a global option and doesn't affect functionality (a bit like `debug` maybe).

Context objects with many veriables are almost like singletons. At some point it also becomes opaque which option is used by which part of the system.