Parse markdown properly with markdown-it-py

Use markdown-it-py to parse markdown and pygments to parse code blocks

• Handle tables (resolve #2 (closed)), nested lists;
• Really resolve #1 (closed), use whitespaces properly. html_block tokens are not parsed, but extracted as-is into PO, so shortcodes should not be put in html_blocks;
• Extract messages with correct line numbers;
• Handle code blocks' comments properly