Option to pass initial metadata

[dylanross3]

Related to nteract#4377, it would be nice to be able to pass in initial metadata, something like this:

dx(df, metadata={
  'view': 'scatter',
  'selectedMetrics': [
    'Economy (GDP per Capita)',
    'Family',
    'Health (Life Expectancy)',
    'Freedom'
  ],
  'chart': {
    'metric1': 'Happiness Score',
    'metric2': 'Economy (GDP per Capita)',
    'metric3': 'Freedom',
    'dim1': 'Region',
    'dim2': 'Region'
  }
})