Add support for option system environment to fully support spring properties file

[Thecarisma]

ip = ${IP:127.0.0.1}

[Thecarisma]

No need, allow client to manage using the listener to process before adding to konfiger

[Thecarisma]

Add examples to include how to use the listener to process complex key value file like spring application.properties

[Thecarisma]

Provide util function to expand variable in ${} in example

[Thecarisma]

Accept the variables container as map implementation to allow customizable implementation.