add JsonOutputFormatter

[TomasVotruba]

Closes #1455

Example output

bin/rector p src/Console/Output -n -o json

(-n = --dry-run, -o = --output-format)

{
    "meta": {
        "version": "0.5.x-dev@adba626",
        "config": "/var/www/rector/rector.yaml"
    },
    "totals": {
        "changed_files": 1,
        "removed_and_added_files_count": 0,
        "errors": 0
    },
    "file_diffs": [
        {
            "file": "/var/www/rector/src/Console/Output/JsonOutputFormatter.php",
            "diff": "--- Original\n+++ New\n@@ -25,7 +25,7 @@\n      */\n     private $configuration;\n \n-    public function __construct(\\Symfony\\Component\\Console\\Style\\SymfonyStyle $symfonyStyle, Configuration $configuration)\n+    public function __construct(SymfonyStyle $symfonyStyle, Configuration $configuration)\n     {\n         $this->symfonyStyle = $symfonyStyle;\n         $this->configuration = $configuration;\n",
            "applied_rectors": [
                "Rector\\CodingStyle\\Rector\\Namespace_\\ImportFullyQualifiedNamesRector"
            ]
        }
    ],
    "changed_files": [
        "/var/www/rector/src/Console/Output/JsonOutputFormatter.php"
    ]
}