Custom strings + improvements round

[DannyBen]

This started as a custom strings PR and evolved a little, so:

Improvements:

• Extract all hardcoded strings to a locale-like config, customizable by the user - closes Make all hard coded strings configurable #5
• Add custom-strings example
• Change many bash echo to echo -e to allow the user to apply newlines and more
• Add bashly add strings command, which will copy the strings config to the user space. This command will be the home for additional goodies that can be added to the project.
• Extracted base command to a base class
• Add AssetHelper module, to help replace all those File.expand_path to find things we need to copy.
• Improve integration tests by disabling leeway diff unless we are on CI.

Todo:

• Back to 100% coverage