add ability to specify output path, keep existing behavior

[hthetiot]

• Add --help option
• Ability to specify resource output path via --path
• Ability to specify resource icon output path via --icon-dir

Example:

cordova-icon --icon assets/MyIcon-Icon-blue.png --path Files will be in config/res/icon/ folder.

cordova-icon --icon assets/MyIcon-Icon-blue.png --path assets --icon-dir icon-blue Files will be in assets/icon-blue/ folder.

Based on work of @miraculixx via #70 but keep existing behavior instead of original PR and is rebase on master.

cordova-splash equivalent https://github.com/AlexDisler/cordova-splash/pull/48