title: PyLatex How to add custom usepackage
body: <p>i wanted to know which is the command to add custom use packages to pylatex. i have already tried using the document.append function and document.preamble.append function but i am still getting errors.</p>
  
  <pre><code># encoding=utf-8
  
  from pylatex import Document, Section, Subsection, Command
  from pylatex.utils import italic, NoEscape
  from pylatex.package import Package
  from pylatex import Document, Section, UnsafeCommand
  from pylatex.base_classes import Environment, CommandBase, Arguments
  
  
  
  
  
  def fill_document(doc):
  """Add a section, a subsection and some text to the document.
  
  :param doc: the document
  :type doc: :class:`pylatex.document.Document` instance
  """
  with doc.create(Section('A section')):
      doc.append('Some regular text and some ')
      doc.append(italic('italic text. '))
  
      with doc.create(Subsection('A subsection')):
          doc.append('Also some crazy characters: $&amp;#{}')
  
  
  if __name__ == '__main__':
  # Basic document
  doc = Document('basic')
  fill_document(doc)
  
  doc.generate_pdf(clean_tex=False)
  doc.generate_tex()
  
  # Document with `\maketitle` command activated
  doc = Document()
  
  #packages = [Package('caratula',"showRevisiones")]
  
  
  #doc.append(Package("caratula"))
  
  doc.data.append(Package("caratula"))
  
  doc.preamble.append(Command('begin', 'document'))
  doc.preamble.append(Command('materia', 'Materia del Apunte'))
  doc.preamble.append(Command('tipoapunte', str('Tipo de Apunte (Teorico o       Practico)')))
  doc.preamble.append(Command('fecha', NoEscape(r'\today')))
  doc.preamble.append(Command('tema', NoEscape(r'Tema')))
  doc.preamble.append(Command('subtema', NoEscape(r'Subtema')))
  doc.preamble.append(Command('autor',["Apellido", "Nombre2", "002/01","email2@dominio.com"]))
  doc.preamble.append(Command('revision',["05/01/2015", "Apellido, Nombre2", "em"]))
  doc.append(NoEscape(r'\maketitle'))
  
  fill_document(doc)
  
  doc.generate_pdf('basic_maketitle', clean_tex=False)
  
  # Add stuff to the document
  with doc.create(Section('A second section')):
      doc.append('Some text.')
  
  doc.generate_pdf('basic_maketitle2', clean_tex=False)
  tex = doc.dumps()  # The document as string in LaTeX syntax
  
  
  import subprocess
  subprocess.Popen("basic_maketitle2.tex",shell=True)
  </code></pre>