Add support of the global configuration

[francoispluchino]

The new location of the configuration has been moved to the config section of the local composer.json file. It would be interesting to be able to configure the plugin globally, thus avoiding to configure the plugin for each project.

Example:

In local composer.json or ~/.composer/config.json or ~/.composer/composer.json:

{
    "config": {
        "fxp-asset": {
            "pattern-skip-version": "(-patch)"
        }
    }
}

[francoispluchino]

Added by 7ab02bc.