Do not indentation on the contents of `<textarea>`

[ota-meshi]

Tell us about your environment

• ESLint version: 5.2.0
• eslint-plugin-vue version: 5.0.0-beta.5
• Node version: v10.13.0

Please show your full configuration:

{
  "rules": {
    "vue/html-indent": "error"
  }
}

What did you do?

<template>
  <textarea>
a
b</textarea>
</template>

What did you expect to happen?

Do not report indentation errors on the contents of <textarea>.

What actually happened?

Expected indentation of 4 spaces but found 0 spaces. [vue/html-indent] (3, 1)
Expected indentation of 4 spaces but found 0 spaces. [vue/html-indent] (4, 1)