Intellicode/eslint-plugin-react-native

# Detect unused StyleSheet rules in React components
When working on a component over a longer period of time, you could end up with unused StyleSheet rules that slipped in over time but are forgotten as you continue to improve your UX/UI design.

## Rule Details

The following patterns are considered warnings:

```js
const styles = StyleSheet.create({
  text: {}
});

const Hello = React.createClass({
  render: function() {
    return <Text>Hello {this.props.name}</Text>;
  }
});
```

The following patterns are not considered warnings:
```js
const styles = StyleSheet.create({
  name: {}
});

const Hello = React.createClass({
  render: function() {
    return <Text style={styles.name}>Hello {this.props.name}</Text>;
  }
});
```
The most common case.

```js
const styles = StyleSheet.create({
  text: {}
});
const Hello = React.createClass({
  propTypes: {
    textStyle: Text.propTypes.style
  },
  render: function() {
    return <Text style={[styles.text, textStyle]}>Hello {this.props.name}</Text>;
  }
});
```
Style rules referenced in a Style array are marked as used.

```js
const styles = StyleSheet.create({
  name: {}
});

const Hello = React.createClass({
  render: function() {
    return <Text textStyle={styles.name}>Hello {this.props.name}</Text>;
  }
});
```
Style rules referenced in a conditional and logical expressions are marked as used.

```js
const styles = StyleSheet.create({
  name: {}
});

const Hello = React.createClass({
  getInitialState: function() {
    return {condition: true};
  },

  render: function() {
    return <Text textStyle={[this.state.condition && styles.name]}>
      Hello {this.props.name}
    </Text>;
  }
});
```

```js
const styles = StyleSheet.create({
  name: {},
  alternate: {},
});

const Hello = React.createClass({
  getInitialState: function() {
    return {condition: true};
  },

  render: function() {
    return <Text textStyle={[this.state.condition ? styles.name : styles.alternate]}>
      Hello {this.props.name}
    </Text>;
  }
});
```
Rules are also marked as used when they are used in tags that contain the word `style`.

```js
const styles = StyleSheet.create({
  name: {},
  welcome: {}
});
const Hello = React.createClass({
  render: function() {
    return <Text style={styles.name}>Hello {this.props.name}</Text>;
  }
});
const Welcome = React.createClass({
  render: function() {
    return <Text style={styles.welcome}>Welcome</Text>;
  }
});
```
Usage is tracked over multiple components in the same file.

```js
const styles = StyleSheet.create({
  text: {}
});
```
There should be at least one component, so centralized `StyleSheets` are not checked for unused rules.