Added options to make Robodoc more customizable.

Added options to make Robodoc more customizable.

Added --module_index_menu to generate module index menu on a module
page and to generate header index menu on a header page.

Added --document_header option to specify file where to read document
header.

Added --document_footer option to specify file where to read document
footer.

Made other minor modifications mainly to the HTML generator.