feat(markdown): provide default blockquote css

```sass
blockquote
    position: relative
    margin-left: 0
    +py($md)
    +px($xl)
    &:before,&:after
      position: absolute
      top: 0
    &:after
      content: '“'
      left: 0.3em
      font-size: 2em
      line-height: 1.4em
    &:before
      content: ''
      left: 0
      width: 0.4em
      height: 100%
      background: currentColor
      opacity: 0.1
```