Here's my solution, which works with Jekyll 3.1.3 and GitHub Pages as of today. Like some solutions others have given, it simply inspects the page's URL and builds the breadcrumbs from it.
{% unless page.hide-breadcrumbs %}
- {{site.title}}
{% assign crumbs = page.url | split: '/' %}
{% for crumb in crumbs offset:1 %}
-
{% unless forloop.last %}
{{ crumb | capitalize }}
{% else %}
{{ crumb | capitalize }}
{% endunless %}
{% endfor %}
{% endunless %}