Provide styles to normalize drop caps

[JayPanoz]

This is part of the accessibility baseline, as defined by issue #18

We currently normalize everything but drop caps when the a11y flag is set, removing italics, superscripts, etc.

Drop caps are quite popular in ebooks (for some unknown reason 😛) and their implementation can vary greatly so we need to find some reliable way to normalize them e.g. if using p:first-letter, it won’t override span.dropcap so we need to go the extra mile, it’s impossible to override svg and img, etc.

This normalization is bound to fail with at least some implementations.