Support custom commands in 'conan.config.json'

[jgsogo]

We already have a conan.config.json file we can use to add custom commands to the extension. The user would be able to:

• associate custom commands per project-configuration
• override install command per project-configuration
• use config options like $(conan), $(install-path), $(conanfile),...
• (Not sure:) accumulate custom commands from every matching <project-pattern>::<configuration> (order matters!)

{
  "commands": {
    "<project-pattern>::configuration": {
      "My command": "do wathever thing",
      "Other command": "do wathever thing"
    },
    "ThisProject::Release|Win32": {
      "install": "$(conan) config install $(conanfile) -r myremote",
      "My command": "different for this project"
    }
  }
}

The UI should be quite straightforward using the context menu:

[Project (right click)]
+-- Install (Current project)  // This will run the `install` command
+-- Conan
    +-- My command
    +-- Other command

The main problem I can think about is that we should recreate the menus with every configuration change, although it might not be a bid deal.

wdyt?