Add expandable arrays

The expandable arrays allow parsing nested arrays. For example:

['tr', [['td', '0'], ['td', '1'], ['td', '2']]]
['tr', (['td', "#{i}"] for i in [0..2])]

Before, the same could be achieved as follows:

['tr'].concat([['td', '0'], ['td', '1'], ['td', '2']]])
['tr'].concat(['td', "#{i}"] for i in [0..3])

The syntax without concat looks more natural and elegant.