enable backtick_code_block to parse 'start:'

Signed-off-by: Jincheng Miao <jincheng.miao@gmail.com>
enable backtick_code_block to parse 'linenos:'

Signed-off-by: Jincheng Miao <jincheng.miao@gmail.com>
set 'linenos' to 'true' when 'start' is given

Signed-off-by: Jincheng Miao <jincheng.miao@gmail.com>