Add formatting options for manuscript builds

[vkbo]

Summary:

This PR adds the following:

• A separate setting for preserving tabs in HTML builds that is off by default. Tabs do not generally render in HTML.
• A setting that disables hard line breaks in paragraphs for Markdown builds. By default, such breaks are preserved.
• Settings to centre title, chapter and scene headers for novel documents.
• Settings to add page break before title, chapter and scene headers for novel documents,
• Allows special titles #! also in notes.
• Allows excluding specific keywords when keywords are included in the manuscript.

Related Issue(s):

Closes #944
Closes #1117
Closes #1132
Closes #1661

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed