Global composer support

[ghost]

This is a feature request to enable global usage of the plugin. With this you can install the plugin globally and it will display the following behavior:

• if plugin is installed in local project global shared-package settings will not be merged
• if plugin is not installed in local project global shared-package settings will be merged
• if plugin is installed in local project and no vendor-dir is defined the plugin will not activate
• if plugin is installed in global but not local and no vendor-dir is defined the plugin will not activate

The last two changes in behavior just create more flexibility for the future. We should not fail the install when the plugin is not able to perform.

For this the tests should be adapted to the new behavior.