Rename defaultValue/Checked to initialValue/Checked

English is not my native language so I could be off here. But it seems there is a seemingly very intentional distinction between `initial` (state) and `default` (props) in React. `defaultValue` seems incorrect for inputs as the property does not represent the default value for when no value is given, but only the initial value (state) of the input. Nor can it be used as a default value for when `value` is not set.

The implementation even looks like this (`getInitialState`, `initialValue`, etc):
```
  getInitialState: function() {
    var defaultValue = this.props.defaultValue;
    return {
      initialChecked: this.props.defaultChecked || false,
      initialValue: defaultValue != null ? defaultValue : null
    };
  },
```

I realize this is a breaking change, but I feel like it sets an important precedent for what is the "correct" naming of properties. I also imagine the surface area should be rather small and very easily fixed for those affected, just search-replace (or very easy to codemod).

cc @sebmarkbage @zpao