fix(cli): show help when no argument is passed

[masahiro331]

Trivy version

$ ./trivy
VERSION:   dev

$ git rev-parse HEAD
2231e4068d5b6f7d0f2707d8669d6f906fc4ebaa

PR Description

1. Trivy client command has bug. If run arguments empty, trivy client command put error.

$ trivy c
2020-09-07T23:53:14.510+0900    FATAL   failed to initialize options: no target is specified

fix that

$ trivy c 
NAME:
   trivy - A simple and comprehensive vulnerability scanner for containers

USAGE:
   trivy [global options] command [command options] target

VERSION:
   dev

COMMANDS:
   image, i          scan an image
   filesystem, fs    scan local filesystem
   repository, repo  scan remote repository
   client, c         client mode
   server, s         server mode
   help, h           Shows a list of commands or help for one command

GLOBAL OPTIONS:
   --quiet, -q        suppress progress bar and log output (default: false) [$TRIVY_QUIET]
   --debug, -d        debug mode (default: false) [$TRIVY_DEBUG]
   --cache-dir value  cache directory (default: "/Users/masahiro331/Library/Caches/trivy") [$TRIVY_CACHE_DIR]
   --help, -h         show help (default: false)
   --version, -v      print the version (default: false)

2. Changed so that no error is output when file system and repository commands execute help.

[codecov]

Codecov Report

Merging #628 into master will decrease coverage by 0.21%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
- Coverage   54.84%   54.63%   -0.22%     
==========================================
  Files          48       48              
  Lines        2299     2297       -2     
==========================================
- Hits         1261     1255       -6     
- Misses        908      910       +2     
- Partials      130      132       +2     

Impacted Files	Coverage Δ
internal/config/artifact.go	66.66% <40.00%> (-22.23%)	⬇️
internal/artifact/config/config.go	86.20% <100.00%> (-0.46%)	⬇️
internal/client/config/config.go	82.85% <100.00%> (-0.48%)	⬇️

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