-o json contains non-json formatted output

When specifying JSON output, kubeval prints any warnings and errors at the top of the output in plain text, making it difficult to parse the output programatically.

eg:
```
c:\> kubeval -d .\folder --ignore-missing-schemas -o json
WARN - Set to ignore missing schemas
[
{
"filename": "folder\\subfolder\\configfile.yaml",
"kind": "Namespace",
"status": "valid",
"errors": []
}
]
```

Suggestion to either remove these from the output, or add them to the output as native JSON, eg:

```
{
"Warnings: [{"msg" : "Set to ignore missing schemas"],
"Errors": [],
"Results" : [
{
"filename": "folder\\subfolder\\configfile.yaml",
"kind": "Namespace",
"status": "valid",
"errors": []
}
]
}
```

Version information:
```
Version: 0.15.0
Commit: df50ea7fd4fd202458002a40a6a39ffbb3125bad
Date: 2020-04-14T09:32:29Z
```