Added nested JSON object functionality to group directive

[ginogervasio]

Hi @mkasa - Really awesome code. I just wanted to show you something I had to do for my side project in case you were open to the suggestion/found it useful. I basically needed to make a few changes to your select_json function to do two things - 1. Return an empty string instead of raising an error to make the schema a bit more lenient and 2. Allow the group directive to handle a nested JSON object so that:

$ cat test.json
{"name": "John", "personal_info": {"age": 30, "sex": "male"} }
$ cat test.ts
table { "name"; group "personal_info" { "age", "sex"; } }
$ json2xlsx test.ts -j test.json -o output.xlsx

Looking forward to hearing from you about this.

[AdmAlexus]

Raise it's PR. Very much needed functionality
@mkasa