cb_page_gen feature / logic to support multiple metadata files

[evanwill]

Can _config.yml metadata value support multiple metadata files? metadata: items_2; items_3

• [ ] In template site.data[site.metadata] would need to be replaced with something that would always combine the multiple csvs. Could be done using cb_helpers fairly easily, creating a new site variable to replace site.data[site.metadata] that is the combined arrays--but this couldn't be replicated in CB-GH, would probably involve some ugly liquid.
• [ ] cb_page_gen default would have to combine the multiple csvs (easy).

[evanwill]

see #34 use case

[evanwill]

in template, possible liquid solution for combining the files:

{% assign metadata = site.metadata | split: ';' %}
{% for m in metadata %}
{% assign items = items | concat: site.data[m] %}
{% endfor %}