Extend the code snippet included in the issue and refactored how the code snippet is printed

[ccojocar]

fixes #268

This will capture 3 lines around the place where the issue occurs. The output looks something like:

[/Users/cosmin/go/src/github.com/securego/gosec/issue.go:180] - G307 (CWE-703): Deferring unsafe method "Close" on type "*os.File" (Confidence: HIGH, Severity: MEDIUM)
    179:        if file, err := os.Open(fobj.Name()); err == nil {
  > 180:                defer file.Close()
    181:                s := codeSnippetStartLine(node, fobj)



Summary:
   Files: 50
   Lines: 8069
   Nosec: 21
  Issues: 1

Also it marks properly a multiline finding:

[/Users/cosmin/go/src/github.com/securego/gosec/issue.go:170-171] - G102 (CWE-200): Binds to all network interfaces (Confidence: HIGH, Severity: MEDIUM)
    169: func multiline() {
  > 170:        _, _ = net.Listen("tcp",
  > 171:                "0.0.0.0:2000")
    172: }

In this way, it seems easier to figure out the place where the security issue is detected.

[codecov-commenter]

Codecov Report

Merging #497 into master will increase coverage by 0.94%.
The diff coverage is 92.00%.

@@            Coverage Diff             @@
##           master     #497      +/-   ##
==========================================
+ Coverage   71.75%   72.70%   +0.94%     
==========================================
  Files           9        9              
  Lines         662      674      +12     
==========================================
+ Hits          475      490      +15     
+ Misses        164      163       -1     
+ Partials       23       21       -2     

Impacted Files	Coverage Δ
issue.go	75.00% <92.00%> (+11.53%)	⬆️

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[gcmurphy]

Looks good.

(I didn't check if this caused any issues with other output formatters though)

[ccojocar]

on other format it generates something like this:

{
        "Golang errors": {},
        "Issues": [
                {
                        "severity": "MEDIUM",
                        "confidence": "HIGH",
                        "cwe": {
                                "ID": "703",
                                "URL": "https://cwe.mitre.org/data/definitions/703.html"
                        },
                        "rule_id": "G307",
                        "details": "Deferring unsafe method \"Close\" on type \"*os.File\"",
                        "file": "/Users/cosmin/go/src/github.com/securego/gosec/issue.go",
                        "code": "180: \tif file, err := os.Open(fobj.Name()); err == nil {\n181: \t\tdefer file.Close()\n182: \t\ts := codeSnippetStartLine(node, fobj)\n",
                        "line": "181",
                        "column": "3"
                }
        ],
        "Stats": {
                "files": 50,
                "lines": 8093,
                "nosec": 21,
                "found": 1
        }
}```

[ccojocar]

3 lines are taken from the code snippet and inclined with the line numbers.