Initial support for symfony 4

[piotrkwiecinski]

I did a manual test via composer using below global composer.json:

{
    "require": {
        "staempfli/magento2-code-generator": "^1.10",
	"staempfli/universal-code-generator": "dev-symfony4 as 1.2.x-dev",
	"symfony/console": "~4.1"
    },
    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/piotrkwiecinski/universal-code-generator.git"
        }
    ]
}

Package operations: 10 installs, 0 updates, 0 removals
  - Installing composer/ca-bundle (1.1.4): Loading from cache
  - Installing padraic/humbug_get_contents (1.1.2): Loading from cache
  - Installing symfony/polyfill-ctype (v1.10.0): Loading from cache
  - Installing symfony/yaml (v4.2.3): Loading from cache
  - Installing symfony/polyfill-mbstring (v1.10.0): Loading from cache
  - Installing symfony/contracts (v1.0.2): Loading from cache
  - Installing symfony/console (v4.2.3): Loading from cache
  - Installing staempfli/universal-code-generator (dev-symfony4 df680e2): Cloning df680e2c0b from cache
  - Installing padraic/phar-updater (v1.0.6): Loading from cache
  - Installing staempfli/magento2-code-generator (1.11.0): Loading from cache

Generator launched as expected and commands seam to load.

@jalogut @mhauri

Refs: staempfli/magento2-code-generator#21
Refs: #1