Added support for nested objects

[sbrichardson]

Currently, if a css object has nested classes the resulting css will be [object Object]

This PR will process them correctly by recursively calling for any nested objects, building the nested selectors correctly.

Added isPlainObj util function, can modify to something simpler if needed.

[sbrichardson]

Example usage

const sample_with_class = {
  div: {
    display: 'inline-flex',
    appearance: 'none',
    border: 'none',
  },
  '.title': {
    fontSize: 32,
    'font-weight': 700,
    border: '0 solid transparent',
    transformOrigin: 'center top',
    'transform-origin': 'center top',
    perspective: '1em',
  },
} 

const output = objToCss(sample_with_class)

/*

// Result

div:{display:inline-flex;appearance:none;border:none;}.title:{font-size:32px;font-weight:700px;border:0 solid transparent;transform-origin:center top;transform-origin:center top;perspective:1em;}'

// Formatted Result

div: {
  display: inline-flex;
  appearance: none;
  border: none;
}

.title: {
  font-size: 32px;
  font-weight: 700px;
  border: 0 solid transparent;
  transform-origin: center top;
  transform-origin: center top;
  perspective: 1em;
}

*/

[natew]

Sorry, took me a while to see this. This can be done at a higher level with a different package, since I want to just keep this simple. People may want different classnames for example, not sure.