Expose markdown plugin metadata functions to allow for reuse in other plugins

Currently, it's necessary for content plugins to duplicate `parseMetadata` and `extractMetadata` from `MarkdownPage` in order to handle wintersmith metadata. This change exposes `extractMetadata` for reuse in other plugins.

Doing so allows me to solve lhagan/wintersmith-pandoc#4 while eliminating the need for any future plugin changes if the metadata functions change. This worked for me, but I'm open to other options if there's a better way to accomplish this.