added ability to set custom styles

[FezVrasta]

This PR allows 3rd party modifiers to hook inside the applyStyles modifier.

Thanks to this edit, your custom modifier can now apply custom styling to the popper element defining the CSS properties inside data.styles.

Example:

function hidePopperModifier(data) {
  data.styles.display = 'none';
  return data;
}

This should help with #52

[andresgutgon]

👏