Add JSON logger, useful for CI and analyzing results of Infection programmatically

[maks-rafalko]

This PR:

• adds new json report type (file logger)
• covered by tests
• Doc PR: Add information about json logger site#166

Example of report JSON file

{
    "stats": {
        "totalMutantsCount": 10,
        "killedCount": 2,
        "notTestedCount": 2,
        "escapedCount": 2,
        "errorCount": 2,
        "timeOutCount": 2,
        "msi": 60,
        "mutationCodeCoverage": 80,
        "coveredCodeMsi": 75
    },
    "escaped": [
        {
            "mutator": {
                "mutatorName": "PregQuote",
                "originalSourceCode": "<?php $a = 1;",
                "mutatedSourceCode": "<?php $a = 2;",
                "originalFilePath": "foo\/bar",
                "originalStartLine": 9
            },
            "diff": "--- Original\n+++ New\n@@ @@\n\n- echo 'original';\n+ echo 'escaped#1';",
            "processOutput": "process output"
        },
        {
            "mutator": {
                "mutatorName": "For_",
                "originalSourceCode": "<?php $a = 1;",
                "mutatedSourceCode": "<?php $a = 2;",
                "originalFilePath": "foo\/bar",
                "originalStartLine": 10
            },
            "diff": "--- Original\n+++ New\n@@ @@\n\n- echo 'original';\n+ echo 'escaped#0';",
            "processOutput": "process output"
        }
    ],
    "timeouted": [
        {
            "mutator": {
                "mutatorName": "PregQuote",
                "originalSourceCode": "<?php $a = 1;",
                "mutatedSourceCode": "<?php $a = 2;",
                "originalFilePath": "foo\/bar",
                "originalStartLine": 9
            },
            "diff": "--- Original\n+++ New\n@@ @@\n\n- echo 'original';\n+ echo 'timedOut#1';",
            "processOutput": "process output"
        },
        {
            "mutator": {
                "mutatorName": "For_",
                "originalSourceCode": "<?php $a = 1;",
                "mutatedSourceCode": "<?php $a = 2;",
                "originalFilePath": "foo\/bar",
                "originalStartLine": 10
            },
            "diff": "--- Original\n+++ New\n@@ @@\n\n- echo 'original';\n+ echo 'timedOut#0';",
            "processOutput": "process output"
        }
    ],
    "killed": [
        {
            "mutator": {
                "mutatorName": "PregQuote",
                "originalSourceCode": "<?php $a = 1;",
                "mutatedSourceCode": "<?php $a = 2;",
                "originalFilePath": "foo\/bar",
                "originalStartLine": 9
            },
            "diff": "--- Original\n+++ New\n@@ @@\n\n- echo 'original';\n+ echo 'killed#1';",
            "processOutput": "process output"
        },
        {
            "mutator": {
                "mutatorName": "For_",
                "originalSourceCode": "<?php $a = 1;",
                "mutatedSourceCode": "<?php $a = 2;",
                "originalFilePath": "foo\/bar",
                "originalStartLine": 10
            },
            "diff": "--- Original\n+++ New\n@@ @@\n\n- echo 'original';\n+ echo 'killed#0';",
            "processOutput": "process output"
        }
    ],
    "errored": [
        {
            "mutator": {
                "mutatorName": "PregQuote",
                "originalSourceCode": "<?php $a = 1;",
                "mutatedSourceCode": "<?php $a = 2;",
                "originalFilePath": "foo\/bar",
                "originalStartLine": 9
            },
            "diff": "--- Original\n+++ New\n@@ @@\n\n- echo 'original';\n+ echo 'error#1';",
            "processOutput": "process output"
        },
        {
            "mutator": {
                "mutatorName": "For_",
                "originalSourceCode": "<?php $a = 1;",
                "mutatedSourceCode": "<?php $a = 2;",
                "originalFilePath": "foo\/bar",
                "originalStartLine": 10
            },
            "diff": "--- Original\n+++ New\n@@ @@\n\n- echo 'original';\n+ echo 'error#0';",
            "processOutput": "process output"
        }
    ],
    "uncovered": []
}x

Can be useful for CI and any other place where results of Mutation Testing need to be analyzed programmatically.

Personally, I'm building something new for Infection, so this report covers 100% of my needs. This report is used to build such UI

[maks-rafalko]

Added docs.

Thank you, @sanmai.