Weights and styles of standard fonts should not be their own families

[jurgenbelien]

Currently, setting just the weight or style of a standard font fails, because they are defined as their own families:

export default [
  'Courier',
  'Courier-Bold',
  'Courier-Oblique',
  [...]
];

It would be helpful if weights and styles are all registered under the correct family, like so:

<Text style={{fontFamily: "Helvetica"}}>
  This is an example of 
  <Text style={{fontStyle: "italic"}}>Italic text</Text>
</Text>

Instead of:

<Text style={{fontFamily: "Helvetica"}}>
  This is an example of 
  <Text style={{fontStyle: "Helvetica-Oblique"}}>Italic text</Text>
</Text>

[diegomura]

Keeping #649 to track this. I'll try to get into this soon