Add hook for rendering of custom <head> content

This allows layouts and templates to hook into providing their own
<head> element content for the application layout using
CaptureHelper#content_for[1].

[1]: http://api.rubyonrails.org/classes/ActionView/Helpers/CaptureHelper.html#method-i-content_for
Add <link> tag to RSS feed on posts root

This allows browsers and feed readers to auto-detect the RSS feed.

See AssetTagHelper#auto_discovery_link_tag[1].

[1]: http://api.rubyonrails.org/v4.1.0/classes/ActionView/Helpers/AssetTagHelper.html#method-i-auto_discovery_link_tag
Add controller test, assert inclusion of RSS <link>

Per @steveklabnik’s [suggestion][c] to add the previously in-line [pull
request test][prt] to the project.

[c]: https://github.com/artisan-tattoo/artisan_assistant/pull/10#issuecomment-46135726
[prt]: https://gist.github.com/andrewsardone/66f8dacafb0e63feb065
Simplify RSS <link> test w/ assert_select

No need to mess with Nokogiri -- we can just use [assert_select][as].

[as]: http://cl.ly/2Q0Z2s3h3M2s
Replace RSS <link> controller test w/ view test

Per [a suggestion][c] by @steveklabnik. The view test seems to involve a
little more awkward setup (stubbing out @posts and current_shop) and I’m
addmittedly unexperienced with best practices around raisl view testing,
but I suppose a ActionView::TestCase is a little more targeted.

[c]: https://github.com/artisan-tattoo/artisan_assistant/pull/10#commitcomment-6686554
Merge pull request #10 from andrewsardone/rss_link

Add <link> tag to RSS feed on posts root