added font arg in HTMLMixin component

[boludo00]

This change allows you to supply a font argument when invoking the write_html method since it defaults to Times.

 class MyFPDF(FPDF, HTMLMixin):
     pass
 pdf = MyFPDF()
 pdf.add_page()
 pdf.write_html("THIS BETTER BE Arial and NOT Times", font_face="arial")
 pdf.output("simple.pdf")

[Eddiedigits]

What about font_size and line height?

[Lucas-C]

You may want to submit this PR against fpdf2, an actively maintained fork of pyfpdf.