<base href="../"> messes with anchor links

[elinorbgr]

Symptom:

Go to http://azerupi.github.io/mdBook/cli/build.html, click on the title "The build command". You are redirected to the the introduction page. I'd expect this link to just be an anchor on the same page.

This is most likely due to the <base href="../"> at the top of the page.

[azerupi]

@steveklabnik could this have been introduced by your PR #207? I am 90% confident this worked before. :)

[steveklabnik]

Ah, I think it was ☹️

I won't have time to look into this until tomorrow, but I'll fix it.