Fix bug for nested elements in pre block text

Fixing bug for pre block text where nested elements display poorly because they're missing a new line.

Example:

• Currently, this JSON example with nested elements in pre block will be formatted poorly in Slack with all the text on one line.

{  "name": "slackify-html",  "version": "1.0.0",  "description": "convert simple html to slack markdown",  "main": "slackify-html.js",  "scripts": {    "test": "tap tests.js"  }}

• With this fix in place, this same example will display in Slack with the text of the nested elements separated by new lines.

{
  "name": "slackify-html",
  "version": "1.0.0",
  "description": "convert simple html to slack markdown",
  "main": "slackify-html.js",
  "scripts": {
    "test": "tap tests.js"
  }
}