[Plan] Add subcommand 'fetch' for copying files or dirs from target hosts to local about gossh HOT 2 CLOSED

$ gossh fetch -h

Copy files/dirs from target hosts to local.

Usage:
  gossh fetch [flags]

Examples:

  # Copy host1:/path/foo to local /tmp/backup/host1/path/foo.
  $ gossh fetch host1 -f /path/foo -d /tmp/backup

  # Copy files and dirs from target hosts to local dir /tmp/backup/,
  # and files in local dir /tmp/backup/ would be:
  #    /tmp/backup/host1/path1/foo.txt
  #    /tmp/backup/host1/path2/bar/
  #    /tmp/backup/host2/path1/foo.txt
  #    /tmp/backup/host2/path2/bar/
  #
  $ gossh fetch host1 host2 -f /path1/foo.txt -f /path2/bar/ -d /tmp/backup
    or
  $ gossh fetch host1 host2 -f /path1/foo.txt,/path2/bar/ -d /tmp/backup

Flags:
  -d, --dest-path string   local directory that files/dirs from target hosts will be copied to
  -f, --files strings      files/dirs on target hosts that to be copied
  -h, --help               help for fetch

Global Flags:
  -k, --auth.ask-pass                  ask for password of login user
  -a, --auth.file string               file containing the credentials (format: "username:password")
  -i, --auth.identity-files strings    identity files (default $HOME/.ssh/{id_rsa,id_dsa})
  -K, --auth.passphrase string         passphrase of the identity files
  -p, --auth.password string           password of the login user
  -u, --auth.user string               login user (default $USER)
      --config string                  config file (default {$PWD,$HOME}/.gossh.yaml)
  -H, --hosts.file string              file containing target hosts (format: one host/pattern per line)
  -L, --hosts.list                     outputs a list of target hosts, and does not do anything else
  -P, --hosts.port int                 port of target hosts (default 22)
  -C, --output.condense                condense output and disable color
  -o, --output.file string             file to which messages are output
  -j, --output.json                    output messages in json format
  -q, --output.quiet                   do not output messages to screen (except error messages)
  -v, --output.verbose                 show debug messages
      --proxy.identity-files strings   identity files for proxy (default same as 'auth.identity-files')
      --proxy.passphrase string        passphrase of the identity files for proxy
                                       (default same as 'auth.passphrase')
      --proxy.password string          password for proxy (default same as 'auth.password')
      --proxy.port int                 proxy server port (default 22)
  -X, --proxy.server string            proxy server address
      --proxy.user string              login user for proxy (default same as 'auth.user')
  -U, --run.as-user string             run via sudo as this user (default "root")
  -c, --run.concurrency int            number of concurrent connections (default 1)
  -l, --run.lang string                specify i18n while executing command (e.g. zh_CN.UTF-8|en_US.UTF-8)
  -s, --run.sudo                       use sudo to execute commands/script
      --timeout.command int            timeout seconds for executing commands/script on each target host
                                       or copying local files and dirs to each target host
                                       or copying files and dirs from each target host to local
      --timeout.conn int               timeout seconds for connecting each target host (default 10)
      --timeout.task int               timeout seconds for the current gossh task