Code blocks support for all formats. Single line break support

[hmalphettes]

Prerequisites

• This pull request fixes a bug.
• This pull request adds a feature.
• This pull request enhances existing functionality.
• This pull request introduces breaking change.

For more information, see the Contributing Guide.

Description

Add support for embedding markdown code blocks in the heredoc of a variable description.

For the markdown table format, the code blocks are formatted as HTML on a single line.
For the markdown document format, the code blocks are kept untouched.

Support for single-line breaks: when the line ends with 2 spaces, place
a single <br> for the markdown table format.
For the other format, make sure to not trim the line and keep the newline in place.

This pull request supports rendering

variable "input-with-code-block" {
  description = <<EOD
This is a complicated one. We need a newline.  
And an example in a code block
\```
default     = [
  "machine rack01:neptune"
]
\```
EOD
  default     = [
    "name rack:location"
  ]
}

Into

| Name | Description | Type | Default | Required |
|------|-------------|:----:|:-----:|:-----:|
| input-with-code-block | This is a complicated one. We need a newline.<br>And an example in a code block<code><pre>default     = [<br>  "machine rack01:neptune"<br>]<br></pre></code> | list | `<list>` | no |

Issues Resolved

No issues were previously filed.

Checklist

Put an x into all boxes that apply:

• I have read the Contributing Guidelines.

Tests

• I have added tests to cover my changes.
• All tests pass when I run make test.

I have done rushed job at keeping the tests suite green after adding a variable to the examples folder. That final pass of sanitization for the document format has a single merit: "it works".
I would not merge it though.

Please let me know if there is sufficient interest for this new feature and I'll put in some effort in refactoring the code.

Documentation

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

Code Style

• My code follows the code style of this project.

[khos2ow]

Thanks @hmalphettes for your PR. I'll review it soon.

[khos2ow]

Thank you @hmalphettes , sorry for the long delay!