Various improvements

Now `json4shell` outputs json instead of python object representation.
`ls2json` also working. 
This is what it could do now:
```bash
[~/dev/json4shell]% ./ls2json | ./json4shell object-keys  | ./json4shell array-sort
[
    ".git", 
    ".gitignore", 
    "LICENSE", 
    "README.md", 
    "docs", 
    "json4shell", 
    "jsonops_tests.py", 
    "lib", 
    "ls2json"
]
```
And much more improvements.