Support for latitude/longitude in profile operations

Profile update operations set and set_once now support passing latitude/longitude for geolocation (see https://help.mixpanel.com/hc/en-us/articles/115004499343#latitude-and-longitude-based-geolocation). These should be passed in the 'modifiers' object param, e.g.:

mixpanel.people.set('billybob', {
  plan: 'premium',
  games_played: 1,
}, {
  $latitude: 40.7127753,
  $longitude: -74.0059728,
});

Event operations like track() already supported latitude/longitude as regular event properties.