scan for plain passwords in git hook

[benammann]

# scan in staged files
git secrets scan

# scan in all files added to the git repository (auto excludes git ignores)
git secrets scan -a

# scan and print each scanned filepath
git secrets scan -v

Note: when scanning there should be used all available / accessable contexts. If there is no access to context there should be a warning and it's secrets are skipped

• implement scan command
• skip non-accessable contexts
• add documentation