v0.4.0: the one that removes Block and Element

breaking changes

this release removes booklit.Block and booklit.Element. these two types of
booklit.Content were cop-outs that only served to allow adding CSS classes to
content that they wrapped. this leaked presentation/templating concerns into
content. you should use booklit.Styled instead.

the one use of booklit.Block, which was to force flow content to be block
content, is now localized to booklit.Styled, which was the only place it was
ever needed. there is a boolean field, Block, which can be set on
booklit.Styled to force it to be block content.