Add support for controlling the indentation of Markdown headers

[khos2ow]

Prerequisites

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• 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

This PR adds support for controlling indentation of Markdown headers by adding --indent flag to markdown document and markdown table commands. The available values are 1, 2, 3, 4, 5, 6, 7 and the default value is 2. Any value outside of this range will default to 2.

Issues Resolved

Fixes #89

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.

[khos2ow]

@metmajer I marked this as WIP because it depends on #119. Although to be fair it's not a hard dependency, it means we can proceed with either one and I'm gonna rebase the other, but in terms of git log history I prefer to proceed with 119 first.

[khos2ow]

According to Markdown guide, it seems the maximum depth of header is 6, and considering that we're going one level deeper when rendering markdown document, I think it'll be a good idea to change the maximum --indent level to 5.

This is an example of content being rendered on GitHub:

What do you think @metmajer ?

[metmajer]

@khos2ow Hmm. What if we support the markdown standard on alignment levels from 0 to 6 and simply adjust the default value for markdown document as needed? This would decouple the implementation from the layout.

[khos2ow]

Hmm, I didn't quite get your point. But to be fair going five levels deep would be more than enough and I really don't think it ever happens for that matter. So instead of adjusting indentation value for one specific layout, we can just easily define a maximum depth of 5 which covers everything.

[metmajer]

Never mind, @khos2ow. Do you as you wish. Do you want to deliver this one before #115?

[khos2ow]

It really doesn't matter. We can proceed with this one first since it's smaller in size.

[metmajer]

Great job @khos2ow and thanks for adding the final update!