Add new API: Sphinx.add_latex_package()

Sphinx.add_latex_package() allows sphinx extensions to add latex packages.
It helps creating extensions for latex (for example, latex-theming)

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extra : amend_source : 06930dce3e3dcbd117a529204aa04749bae775b7
Merged in tk0miya/sphinx (pull request #298)

Add new API: Sphinx.add_latex_package()