Allows the user to set the depth of the child elements of the variable to be output during log output

[zero-plusplus]

I propose a new format for specifying the depth of the child elements to retrieve, such as {VARIABLENAME:DEPTH}.

To solve #146, it is necessary to get all the child elements when the log output, but if the outputted variable contains many child elements, the performance will be greatly affected.

Therefore, I solve this issue by allowing the user to set the depth of the child elements to be retrieved.