Add a 'contains' pseudo-class, used to match against text children of a

node.

@@ -96,6 +96,14 @@ const compileMatcher = (ast: AST, selector: string): MatcherProducer => {
     }
   }
 
+  const getNodeText = (node) => {
+    if (node.type === TEXT_NODE) return node.value
+    if (node.type !== ELEMENT_NODE) return ''
+    const { children } = node
+    if (children.length === 0) return ''
+    return children.map(getNodeText).join('')
+  }
+
   const getAttrValueMatch = (value: string, operator: string = '=', caseSensitive: boolean): AttrValueMatch => {
     if (value === undefined) return (attrValue) => attrValue !== undefined
     if (value[ 0 ] === '"' || value[ 0 ] === '\'') value = value.substring(1, value.length - 1)
@@ -267,6 +275,13 @@ const compileMatcher = (ast: AST, selector: string): MatcherProducer => {
               return nthChildMatcher(context, node, parent, pos + 1, debug || parent?.name === 'body')
             }
           }
+          case 'contains': {
+            const contentValueMatch = getAttrValueMatch(ast.argument, '*=', false)
+            return (context, node, parent, i, debug) => {
+              const nodeText = getNodeText(node)
+              return contentValueMatch(nodeText)
+            }
+          }
           default:
             console.error('pseudo-class', nodeUtil.inspect({ selector, ast }, { depth: null, colors: true }))
             throw new Error(`Unknown pseudo-class: ${ast.name}`)