Add hook to manipulate HTML

[jdillard]

Adds a new configuration option that allows users to specify a Python function to manipulate the HTML (via
BeautifulSoup) before PDF generation.

Usage:

# conf.py
simplepdf_html_hook = "./hooks/pdf_hook.py:customize_html"

# hooks/pdf_hook.py
def customize_html(soup, app):
    # manipulate soup
    return soup

Changes:

• Added simplepdf_html_hook config option
• Added _load_html_hook() and _execute_html_hook() methods
• Refactored _toctree_fix() to work with BeautifulSoup objects directly
• Added documentation in docs/configuration.rst