Use logrus for logging

[temsa]

adds log levels, cleanup a bit stdout and brings new CLI commands for logging:

$> ./gammaray -h
Usage of ./gammaray:
  -image string
    	analyze this docker image
  -log-as-json
    	detailed logs should be formated as JSON if true (default: false)
  -log-file string
    	in which file to put the detailed logs (default ".gammaray.log")
  -log-level string
    	minimal level that should be logged (default "info")
  -only-installed
    	force only installed module checking usage (default false: use it as the main strategy then use other fallbacks)
  -only-package-lock
    	force only <package-lock.json> usage (default false: use it as a fallback)
  -only-yarn-lock
    	force only <yarn.lock> usage (default false: use it as a fallback)
  -path string
    	path to installed Node package (locally or inside the container, depending if 'image' is provided)

[coveralls]

Pull Request Test Coverage Report for Build 83

• 773 of 1019 (75.86%) changed or added relevant lines in 10 files are covered.
• 115 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+1.1%) to 77.911%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
analyzer/analyzer.go	2	3	66.67%
versionformatter/versionformatter.go	19	20	95.0%
pathrunner/pathrunner.go	1	3	33.33%
vulnfetcher/ossvulnfetcher/osindexfetcher.go	6	10	60.0%
docker/docker.go	16	23	69.57%
main.go	16	35	45.71%
yarnlockparser/yarnlockparser.go	680	892	76.23%

Files with Coverage Reduction	New Missed Lines	%
main.go	1	61.25%
vulnfetcher/ossvulnfetcher/osindexfetcher.go	1	72.07%
versionformatter/versionformatter.go	113	77.66%

Totals
Change from base Build 79:	1.1%
Covered Lines:	2081
Relevant Lines:	2671

---

💛 - Coveralls