Fixes html and children rendering and ensures consistent padding

[rafamel]

Fixes:

• Ensures/forces consistent paddings across all li elements so checkboxes and non-checkboxes indentation match.
• Fixes Markdown rendering inside checkboxes list elements (such as links, bold…) - it was using $(element).text() and all the rendered elements were left out.
• Fixes rendering checkboxes on lists inside of a parent checkbox (they weren't rendered).
• Ensures that all style, class, or id attributes that a checkbox list element might have are preserved.

[jperichon]

@LingyuCoder are you planning on merging this? I'm especially waiting for the fix that allows rendering of HTML elements within the list elements.