Refactor Normalizer as a class for configurable whitelists and aliases

This turns Normalizer into a class that stores tag & style whitelists, and aliases.

* Document#addFormat makes sure that format tags, styles and aliases are properly added to the normalizer.
* Prefixed all the internal-only methods of the normalizer with an _.
* Added aliasTags settings to certain formats to configure the aliasing behavior.
* Normalizer.stripWhitespace and Normalizer.stripComments are kept as class methods.
* Formats specify styles as camelCase while dom#styles returns dash-separated, so a manual conversion to camelCase was added in _whitelistStyles.

I know that some module + format internals may be in flux, but for now this has helped me to add formats with new tags successfully (e.g. H1 / H2 / H3).