Add dry-run option for CLI

This pull request adds dry run option for CLI as follows:

```
% java -jar build/libs/groovy-ssh-SNAPSHOT-all.jar
usage: [option...] [-e script-text] [script-filename | --stdin] [script-args...]
 -d,--debug     Set log level to debug.
 -e <arg>       Specify a command line script.
 -h,--help      Shows this help message.
 -i,--info      Set log level to info. (default)
 -n,--dry-run   Do a dry run without connections.
 -q,--quite     Set log level to warn.
    --stdin     Specify standard input as a source.
```