Add debug commands

[vjik]

The following console command to debug should be present:

• Dump config files and their content
• Dump config params
• Test if specific parameter exist

Extracted from https://github.com/yiisoft/yii-debug/issues/202

[tomaszkane]

It will be nice to have better verbose for composer yii-config-rebuild even with -vvv I don't see what configs paths is readed/merged.