Add back blackfriday extensions during Markdown conversion

Modified markdownRender and markdownRenderWithTOC in hugolib/page.go to
use the same flags and extensions as were previously used when we simply
called blackfriday.MarkdownCommon to convert Markdown to HTML.  These
flags/extensions were dropped during the refactor that added the
`.TableOfContents` page variable, and caused features like Markdown
tables to no longer work.

Modified the expected output for TestTableOfContents in page_test.go,
apparently changing the flags/extensions caused an `&mdash;` to become
`&ndash;`.

See discussion in issue #173.