Add extends and block tag. Changed the following files:
M  lib/liquid/block.rb
M  lib/liquid/document.rb
M  lib/liquid/htmltags.rb
M  lib/liquid/standardfilters.rb
M  lib/liquid/tag.rb
M  lib/liquid/tags/assign.rb
M  lib/liquid/tags/capture.rb
M  lib/liquid/tags/case.rb
M  lib/liquid/tags/cycle.rb
M  lib/liquid/tags/decrement.rb
A  lib/liquid/tags/default_content.rb
A  lib/liquid/tags/extends.rb
M  lib/liquid/tags/for.rb
M  lib/liquid/tags/if.rb
M  lib/liquid/tags/include.rb
M  lib/liquid/tags/increment.rb
A  lib/liquid/tags/inherited_block.rb
M  lib/liquid/template.rb