Fix broken relative links

[exoego]

Fixes #188

This PR consists of two fixes:

1. dc004a4 Resolves all relative links in docs/features into full-path from the content root, on build time.
2. ee43f67 Add trailing-slash in feature links on build time, so links work consistently on local and GitHub pages,

I think it is much easier to implement these fixes in this repo rather than in a separate repo (update_docs task).

This fix might be ugly, but it works as far as I tested.
I hope Middleman Ninja refactors someday.

[JonRowe]

Thanks, I was a bit apprehensive about this approach but it seems to work and means we can maintain the gherkin links in their repos.

For future travellers I tested this as follows:

• Replicated the issue locally (you need a trailing slash to make the pages mess up)
• Cherry-picked just the code commits
• Ran my own build and removed some unrelated changes (CNAME, dates in feeds.xml)
• Checked it fixed an originally reported issue
• No diff to this PR so code is as generated by middleman
• Ran the link checker which reports all links work (it did this before due to the lack of trailing slashes locally)