Rule proposal: sorted propTypes

[lencioni]

Similar to #16, some people might might to enforce that propTypes are sorted alphabetically.

Good:

propTypes: {
  aProp: React.PropTypes.object,
  bProp: React.PropTypes.object,
  cProp: React.PropTypes.object,
},

Bad:

propTypes: {
  bProp: React.PropTypes.object,
  aProp: React.PropTypes.object,
  cProp: React.PropTypes.object,
},