Creating placeholder content

[jimafisk]

It would be nice to be able to create n number of placeholder pages in order to quickly mock a test site and run build time benchmarks.

Proposed command format: plenti gen <#> <type>

Example: plenti gen 35 pages

The scaffolding for the placeholder nodes would be based on the type's _blueprint.json file.

[jimafisk]

We already use the new keyword for plenti new site mysite and plenti new type posts. We could reuse that format and make this command plenti new content 3 posts. It should copy the content from _defaults.json and just use the content type name plus an iterator like posts1, posts2, posts3, etc. We should also error handle if someone tries to create new content from a single type.