Support for line numbers of code block

[lightxue]

Hi,

When linenums enabled, the style of code block will be mess up. Because the DOM structure of the code block of pelican is different with octopress, sass/partials/_syntax.scss should match up peilican.

'markdown.extensions.codehilite': {
    'css_class': 'highlight',
    'linenums': True
}

Can you support this? Thanks!