Give an easy syntax to center-align

[riyadh-h]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• Others, please explain:

Resolves #900

Overview of changes:
MarkBind can now process text to be center-aligned using the syntax of markdown-it-center-text.

Anything you'd like to highlight / discuss:

• This enhancement makes use of the original source code for the plugin mentioned above, customized to use Bootstrap's text-center class instead.
• Tests were added and updated accordingly.

Testing instructions:
n/a

Proposed commit message: (wrap lines at 72 characters)

Give an easy syntax to center-align

Text content is centered by wrapping an HTML element around said text
(if it is not contained within one already) and applying some class
(e.g., `text-center`) to it.

Users who frequently use center-aligned text frequently on their 
website(s) would find it tiresome to do so.

Let's add a simple syntax rule to easily mark text to be center-aligned.

Using a syntax rule requires less typing to be done by the users than
using a component markup, for example.

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

[jonahtanjz]

Thanks for working on this @riyadh-h :)

Generally looks good. You might want to update the user guide (maybe under this section) to include the new feature.

One thing to note: you can ignore the changes of the .png files after running npm run updatetest. It is an open issue #1130.

[riyadh-h]

@jonahtanjz my pleasure, and thanks for the feedback! 😄
I'll try to update the user guide and exclude the image files as soon as I can 👍

[jonahtanjz]

Thanks @riyadh-h :)

LGTM 👍