Allow to use text elements for section text field

[cyb-]

What?

Allow to set Section text field with text elements.

Before

section '*Hello* world :wave:'

{
  "type": "section",
  "text": {
    "type": "mrkdwn",
    "text": "*Hello* world :wave:",
    "verbatim": false
  }
}

After

section do
  text do
    plain_text 'Hello world :wave:'
  end
end

{
  "type": "section",
  "text": {
    "type": "plain_text",
    "text": "Hello world :wave:",
    "emoji": true
  }
}

Why?

• Was initially implemented using mrkdwn object when provided as argument.
• Allow to optionally use plain_text object as mentioned in the documentation.