Add support for BreadcrumbList scheme

[mallardduck]

Supporting the BreadcrumbList scheme can be a useful feature as it helps google add a list of breadcrumb links under search results!

As schemify already includes WebPage scheme it may be pretty easy to get this supported.

See: http://schema.org/BreadcrumbList
And: https://developers.google.com/search/docs/data-types/breadcrumbs

[mallardduck]

Looked into this a bit further comparing what's in the Schema.org site and implemented here. I believe this feels like it could still be a useful feature to some users of Schemify, tho I could also see how this feature may fall out of the initial scope of this Plugin.

That said, if you are interested in this feature I'd be happy to take on the work to help implement this. It seems at minimum we would need to add the following Schema classes:

• Intangible
  • ListItem
  • ItemList
    • BreadcrumbList

Otherwise, if you'd prefer not to add these, yet or ever, I can probably whip up a snippet to accomplish this that could be added to themes.

[stevegrunwell]

It looks like Schema's changed a bit of its underlying architecture since I started the plugin, looks like it's time to revisit. BreadcrumbList seems like an immensely-cool and useful schema type to include!