Add anchors to all bullets

This is very similar to the formatting used in bbatsov/ruby-style-guide/pull/338 and was requested by @bbatsov to be applied to this Rails guide as well. Every bullet is anchored with a short `<a name>` tag and includes a superscript `[link]` anchor at the end. 

Much easier the second time around. :)

I find the individual commits easier when it comes to reading diffs (to make sure I'm not sneaking bad advice into this guide). If you'd care you, take a look at https://github.com/rapid7/rails-style-guide/tree/add-anchors-rails to see the individual commits.