Indent classes for nested list items

[cecasey]

• Removed line that was setting the indent value to 0 for every nested list item.
• Added indent class to test for checked/unchecked lists

[coveralls]

Coverage remained the same at 100.0% when pulling aca0595 on cecasey:bugfix/indent-classes into 502a6bb on nozer:master.

[volser]

@cecasey why do we need it? I did not get

[lucasaides]

it would actually help with the issue I opened recently: #96
it would help when you need indent not for nesting lists, but to move the list more to the center/right