Add backticks option to markdown output

[franklin-stripe]

Prerequisites

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

For more information, see the Contributing Guide.

Description

This adds an option for markdown output to include backticks for
variable and output names. This is useful since names may include
underscores, which most markdown renderers treat as emphasis indicators.

Issues Resolved

This is a slightly different twist on handling double underscores: #48

Checklist

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• I have read the Contributing Guidelines.

Tests

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

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.

[metmajer]

Hi @franklin-stripe and thank you for a great looking, first contribution!

To be frank, I do prefer to follow up on @sjauld's work (in progress) in #63. Back-ticking does have its upsides, such as not having to take care of escaping, but it may as well be uneasy on the eye if you browse the table for information and have to search through varying fonts and styles. Cheers!