[HTML] Introducing tag_indents & tag_styles

[Lucas-C]

This is a follow-up of #1066 that also provides the feature requested in #1074

Checklist:

• The GitHub pipeline is OK (green), meaning that both pylint (static code analyzer) and black (code formatter) are happy with the changes of this PR.

• A unit test is covering the code added / modified by this PR

• This PR is ready to be merged

• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder

• A mention of the change is present in CHANGELOG.md

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

Documentation added to docs/HTML.md:

Styling HTML tags globally

The style of several HTML tags (<a>, <blockquote>, <code>, <pre>, <li>, <h1>, <h2>, <h3>...) can be set globally, for the whole HTML document, by passing tag_styles to FPDF.write_html():

from fpdf import FPDF, FontFace

pdf = FPDF()
pdf.add_page()
pdf.write_html("""
  <h1>Big title</h1>
  <section>
    <h2>Section title</h2>
    <p>Hello world!</p>
  </section>
""", tag_styles={
    "h1": FontFace(color=(148, 139, 139), size_pt=32),
    "h2": FontFace(color=(148, 139, 139), size_pt=24),
})
pdf.output("html_styled.pdf")

Similarly, the indentation of several HTML tags (<blockquote>, <dd>, <li>) can be set globally, for the whole HTML document, by passing tag_indents to FPDF.write_html():

from fpdf import FPDF

pdf = FPDF()
pdf.add_page()
pdf.write_html("""
  <dl>
      <dt>Term</dt>
      <dd>Definition</dd>
  </dl>
""", tag_indents={"dd": 5})
pdf.output("html_dd_indented.pdf")

[Lucas-C]

@gmischler & @andersonhc: could you please review this PR?

It introduces some two new "structuring" parameters to HTML2FPDF, so I'd be happy to get your feedback on that 😊

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.07339% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 93.34%. Comparing base (e85ec4e) to head (dfe0f5b).

❗ Current head dfe0f5b differs from pull request most recent head 103708e. Consider uploading reports for the commit 103708e to get more accurate results

Files	Patch %	Lines
fpdf/html.py	86.17%	5 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1124      +/-   ##
==========================================
- Coverage   93.42%   93.34%   -0.09%     
==========================================
  Files          30       30              
  Lines        9130     9179      +49     
  Branches     2071     2089      +18     
==========================================
+ Hits         8530     8568      +38     
- Misses        371      374       +3     
- Partials      229      237       +8     

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[gmischler]

Looks ok, adding flexibility without making them much more complicated.

[Lucas-C]

Looks ok, adding flexibility without making them much more complicated.

Thank you for the feedback!

Merging this now.