has_knitr               Check if the user has the 'knitr' package
                        installed
md_autolink             Markdown autolink
md_blank                Markdown blank line
md_bold                 Markdown bold emphasis
md_bullet               Markdown bullet list
md_chunk                Markdown code block
md_code                 Markdown code span
md_convert              Convert markdown to HTML
md_disallow             Disallow certain raw HTML
md_escape               Backslash escape all punctuation
md_fence                Markdown fenced code block
md_hardline             Markdown hard line breaks
md_heading              Markdown ATX headings
md_image                Markdown image links
md_indent               Markdown indented code block
md_issue                Markdown GitHub issue
md_italic               Markdown italic emphasis
md_label                Markdown link label
md_link                 Markdown inline link (6.6)
md_list                 Markdown generic list
md_order                Markdown ordered list
md_paragraph            Markdown paragraphs breaks
md_quote                Markdown block quotes
md_reference            Markdown link reference
md_rule                 Markdown horizontal rule (4.1)
md_setext               Markdown Setext headings (4.3)
md_softline             Markdown soft line breaks
md_strike               Markdown strikethrough (extension)
md_table                Markdown tables (4.10 extension)
md_task                 Markdown task list (extension)
md_text                 Markdown textual content
