title: Why eslint-plugin-react-hooks doesn't warn when using react hooks conditionally?
body: <p>I'm using react 16.8, and eslint-plugin-react-hooks 1.6.0 .When I use hooks conditionally, I hoped eslint was going to warn me. Here is my config :</p>
  
  <pre><code>"eslintConfig": {
      "parser": "babel-eslint",
      "plugins": [
        "react-hooks"
      ],
      "rules": {
        "react-hooks/rules-of-hooks": "error",
        "react-hooks/exhaustive-deps": "warn"
      }
  }
  </code></pre>
  
  <p>If I use hooks conditionally in custom hooks, there will be a warning like this: "React Hook \"useState\" is called conditionally. React Hooks must be called in the exact same order in every component render."</p>
  
  <pre><code>function useCustomHook() {
    if (Math.random() &gt; 0.5){
      const [a, setA] = useState(0)
    }
  }
  </code></pre>
  
  <p>but if I use hooks in function component, it does't work.</p>
  
  <pre><code>function MyComponent() {
    if (Math.random() &gt; 0.5){
      const [a, setA] = useState(0)
    }
    return null
  }
  </code></pre>