Introduce composer plugin

[veewee]

This PR will make it possible to install the mago binaries from within composer.
It contains:

• A composer command: composer mago:install-binary that can be used to install the binary for currently required mago version for current platform.
  • The downloaded files will be placed in the vendor directory of this package inside composer/bin/{version}
  • It will also create a .platform file that contains a relative link to the mago executable for current platform.
• Event listeners to download the assets when the package is updated / installed through composer.

This package requires activation of the plugin:

    "config": {
        "allow-plugins": {
            "carthage-software/mago": true
        }
    }

Once the assets are in place, that is where the composer plugin stops.

Next up is a mago PHP binary that will be symlinked in vendor/bin/mago.
This script will:

• Load the current platform executable from the .platform file
• Executes the mago command in a separate processs
• Currently open stdin, stdout and stderr are being linked to the executable, making it possible to use it just as you would in a regular command.

This setup allows to run mago on multiple platforms (docker, virtualbox, host, ...) just as one would expect from regular PHP tools.

[veewee]

I've kept this file for a few reasons:

• Composer adds a binary proxy wrapper. It somehow expects this to be a PHP file.
• In windows, the executable will be suffixed with .exe. I'm not sure if it runs without that suffix.
• This gives us some control about the availability of the executable. For example when composer runs with no-scripts