Add JSON exporting support
Would it be possible to add support for exporting JSON?  

The `toml` export wouldn't be hard to convert from, but JSON is already supported for sharing themes by the implementation of ImGui I'm looking at.

It would look like:

```
{
  "Author": "[optional]",
  "Description": "[optional]",
  "Colors": {
    "Success": [0, 1, 0, 1],
    "Warning": [1, 1, 0, 1],
    ...
  },
  "Options": {
    "Alpha": 1.0,
    "DisabledAlpha": 0.6,
    ...
  }
}
```

Thanks for the great tool!