Accept reports on the stdin

[Nr18]

When you want to scan let say a CDK application using cfn-guard. You first need to find each template and then scan it:

cdk synth
find ./cdk.out -name "*.template.json" \
  -exec cfn-guard validate \
  --rules my-rules.guard \
  --data {} \
  --output-format json \
  --show-summary none > reports/cfn-guard.json \;
report2junit --source-type cfn-guard reports/cfn-guard.json

This would result in a single JSON file that contains 2 reports. The problem with this is that it is not valid JSON as each report is just appended on the next line.

Therefore, it would be easier to accept the report from the stdin. For example:

cdk synth
find ./cdk.out -name "*.template.json" \
  -exec cfn-guard validate \
  --rules my-rules.guard \
  --data {} \
  --output-format json \
  --show-summary none | report2junit ./report/cfn-guard.xml \;

It should just append the report to the ./report/cfn-guard.xml file. This feature will depend on #7 and #8.