feat(toHaveStyleRule): add support for component modifiers

Augments the modifier syntax to allow nested component rules such as
```
const Child = styled.div`
  color: blue;
`;

const Parent = styled.div`
  display: flex;

  ${Child} {
    flex: 1;
  }
`;

fixes #64

src/toHaveStyleRule.js

@@ -1,4 +1,5 @@
 const { getCSS } = require('./utils')
+const { isStyledComponent } = require('styled-components')
 
 const shouldDive = node =>
   typeof node.dive === 'function' && typeof node.type() !== 'string'
@@ -87,9 +88,20 @@ const getDeclaration = (rule, property) =>
 const getDeclarations = (rules, property) =>
   rules.map(rule => getDeclaration(rule, property)).filter(Boolean)
 
+const normalizeModifierOption = (modifiers = []) =>
+  modifiers
+    .map(
+      modifier =>
+        isStyledComponent(modifier)
+          ? `.${modifier.styledComponentId}`
+          : modifier
+    )
+    .join('')
+
 function toHaveStyleRule(received, property, value, options = {}) {
   const classNames = getClassNames(received)
   const ast = getCSS()
+  options.modifier = normalizeModifierOption([].concat(options.modifier))
   const rules = getRules(ast, classNames, options)
 
   if (!rules.length) {

test/toHaveStyleRule.spec.js

@@ -257,3 +257,58 @@ test('selector modifiers', () => {
     modifier: '.parent &',
   })
 })
+
+test('component modifiers', () => {
+  const Text = styled.span`
+    color: grey;
+  `
+
+  const Link = styled.a`
+    color: white;
+
+    ${Text} {
+      color: blue;
+    }
+
+    > ${Text} span {
+      color: green;
+    }
+
+    ${Text} & {
+      color: purple;
+    }
+  `
+
+  toHaveStyleRule(<Link />, 'color', 'white')
+  toHaveStyleRule(<Text />, 'color', 'grey')
+  toHaveStyleRule(
+    <Link>
+      <Text />
+    </Link>,
+    'color',
+    'blue',
+    {
+      modifier: Text,
+    }
+  )
+  toHaveStyleRule(
+    <Link>
+      <Text />
+    </Link>,
+    'color',
+    'green',
+    {
+      modifier: ['> ', Text, ' span'],
+    }
+  )
+  toHaveStyleRule(
+    <Link>
+      <Text />
+    </Link>,
+    'color',
+    'purple',
+    {
+      modifier: [Text, ' &'],
+    }
+  )
+})