Allow multiple mrkdwn elements in <Context>

[yhatt]

<Context> merges continuous text contents into one mrkdwn automatically. However, someone may want to control mrkdwn elements separately for layouting message.

Separated mrkdwn have a look like fields in section block, or flexbox layout in HTML.

[
  {
    "type": "context",
    "elements": [
      {
        "type": "mrkdwn",
        "text": "foo\nbar",
        "verbatim": true
      },
      {
        "type": "mrkdwn",
        "text": "A ->\nB ----->\nC ->\nD -->",
        "verbatim": true
      },
      {
        "type": "mrkdwn",
        "text": "Hello,\nworld!",
        "verbatim": true
      }
    ]
  }
]

Our idea is to allow separated mrkdwn contents by using <div> HTML element. It would allow to use an interesting layout in <Context>.

<Blocks>
  <Context>
    <div>
      First mrkdwn element
    </div>
    <div>
      ┐<br />Second markdown element
    </div>
    <div>
      ┌→<br />┘
    </div>
    <img src="https://placekitten.com/100/100" alt="Kitten" />
  </Context>
</Blocks>

This layout makes sense by treating <Context> block as a flex container.

[yhatt]

I've used <span> instead of <div> to avoid the side effect for layout in HTML-like formatting in #31. <div> is block element so must render as like as block, but <span> inline element may ignore.