feat(scan): add -wordpress-only and -libs-only flag

[kotakanbe]

What did you implement:

Add -libs-only flag that doesn't detect OS packages Vulns to the scan command.

https://vuls.io/docs/en/usage-scan-non-os-packages.html#options

Add -wordpress-only flag that doesn't detect OS packages Vulns to the scan command.

https://vuls.io/docs/en/usage-scan-wordpress.html#scan-wordpress-core-plugin-theme

$ ./vuls scan -h
scan:
        scan
                [-config=/path/to/config.toml]
                [-results-dir=/path/to/results]
                [-log-dir=/path/to/log]
                [-cachedb-path=/path/to/cache.db]
                [-ssh-native-insecure]
                [-ssh-config]
                [-containers-only]
                [-images-only]
                [-libs-only]
                [-skip-broken]
                [-http-proxy=http://192.168.0.1:8080]
                [-ask-key-password]
                [-timeout=300]
                [-timeout-scan=7200]
                [-debug]
                [-pipe]
                [-vvv]
                [-ips]


                [SERVER]...
  -ask-key-password
        Ask ssh privatekey password before scanning
  -cachedb-path string
        /path/to/cache.db (local cache of changelog for Ubuntu/Debian) (default "/home/ubuntu/go/src/github.com/future-architect/vuls/cache.db")
  -config string
        /path/to/toml (default "/home/ubuntu/go/src/github.com/future-architect/vuls/config.toml")
  -containers-only
        Scan running containers only. Default: Scan both of hosts and running containers
  -debug
        debug mode
  -http-proxy string
        http://proxy-url:port (default: empty)
  -images-only
        Scan container images only. Default: Scan both of hosts and images
  -ips
        retrieve IPS information
  -libs-only
        Scan libraries (lock files) specified in config.toml only.
  -log-dir string
        /path/to/log (default "/var/log/vuls")
  -pipe
        Use stdin via PIPE
  -results-dir string
        /path/to/results (default "/home/ubuntu/go/src/github.com/future-architect/vuls/results")
  -skip-broken
        [For CentOS] yum update changelog with --skip-broken option
  -ssh-config
        Use SSH options specified in ssh_config preferentially
  -ssh-native-insecure
        Use Native Go implementation of SSH. Default: Use the external command
  -timeout int
        Number of seconds for processing other than scan (default 300)
  -timeout-scan int
        Number of seconds for scanning vulnerabilities for all servers (default 7200)
  -vvv
        ssh -vvv

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

$ ./vuls scan -libs-only
$ ./vuls scan -wordpress-only

and reporting

Checklist:

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES