Added extrapreamble option

Hello,

this is a proposal in order to add extra text to the preamble. In my case, I need to add several instruccions in the preamble. Here is an example how this works:

    from pylatex import Document
    preamble = r'\setlength\parindent{0pt}'
    doc = Document(extrapreamble=preamble)
    doc.generate_tex()

Then, the output in `default_filename.tex` is:

    \documentclass{article}\usepackage[T1]{fontenc}
    
    \usepackage[utf8]{inputenc}
    
    \usepackage{lmodern}
    \setlength\parindent{0pt}
    \begin{document}\end{document}

I'm open to suggestions.

Felipe