Add more lines in rule code section

[oxdef]

Summary

For more readability it will be great to add more lines in the rule code section (like in Bandit)

Steps to reproduce the behavior

1. Scan some code and get report

Expected behavior

{
		"severity": "HIGH",
		"confidence": "HIGH",
		"rule_id": "G402",
		"details": "TLS InsecureSkipVerify set true.",
		"file": "request.go",
		"code": "122 //some previous code\n 123 InsecureSkipVerify: true\n 124 //...",
		"line": "123"
}

Actual behavior

{
		"severity": "HIGH",
		"confidence": "HIGH",
		"rule_id": "G402",
		"details": "TLS InsecureSkipVerify set true.",
		"file": "request.go",
		"code": "InsecureSkipVerify: true",
		"line": "123"
}

[gcmurphy]

We extract the code segment from the file by using the positional coordinates reported by the AST node here: https://github.com/securego/gosec/blob/master/issue.go#L107

It should be possible to add more context there.