removed an unnecessary blank line
a refectoring of HtmlElement.create()
now you can pass a content as the third argument of HtmlElement.new()
Until today, I haven't noticed that Object#tap was backported to 1.8.7 ...
removed HtmlFormat#configure and use Object#tap instead
use Object#tap in methods of BlockParser
now you can't pass a block to HtmlFormat.create(). Instead, you may use a method chain with #tap
use #tap in TreeStack::Leaf.create()