HTML Reader: support for reading metadata from meta tags

[mfenner]

YAML metadata blocks work well with markdown documents. The standard way to include metadata in HTML documents is tags in the header (using name and content). Could the HTML Reader read in these tags as metadata, using name/content as key/value?

Some of these names will be namespaced, e.g. dc:title or twitter:title. These names could be turned into dc_title and twitter_title variable names.