Add destroy and destroy_all to Perron::Resource

[eelcoj]

Adds destroy and destroy_all methods for deleting resource content files.

Content::Post.find("slug").destroy  # => deletes app/content/posts/slug.md
Content::Post.destroy_all  # => deletes app/content/posts/*.md

To me this is mostly syntactic sugar similar to how Active Record's destroy works (so you do not need to use SQL to delete a table row).

Niche or does it has some good use cases?