Added support for plugin pages
- Now Plugins can add pages directly to Jekyll's site.pages array
  Pages are automatically added from the assets/pages dir
  By default paths are treated as if pages directory is the source dir
  To allow users to configure the destination url:
    1. Add to the yaml front matter, something like url_config: 'some.config.url'
    2. Add a default to the plugin's config.yml a default url:
       some:
         config:
           url: /default/url/

  User facing page customizations:
    1. Configure a page's destination url in the plugin's config
    2. Override the page by placing a page in its destination
    3. Disable a page by setting the url config to false

- Capture tag now supports filters
- New filter join_spaces (cleans up newlines, great for class names)
- _custom directory is now set to plugins dir config so customizations
  and overrides go in _plugins/plugin-name/*