Allow overriding variables in all ogp namespaces

[mrtndwrd]

Under normal circumstances, if you use several namespaces, middleman-ogp will only allow you to override variables in the og namspace by defining them as page variables. Take this index.md as an example:

title: My Index Page
description: My index page looks great!
---
# Welcome

I'm one of those people that writes welcome on their home page.

And let's assume this is our config.rb:

activate :ogp do |ogp|
  ogp.namespaces = {
    og: data.ogp.og,
    twitter: data.ogp.twitter
  }
end

The og tags for this page would look something like:

    <meta property="og:title" content="My Index Page" />
    <meta property="og:description" content="My index page looks great!" />
    <meta property="twitter:description" content="whatever description I have in data/ogp/twitter.yml" />
    <meta property="twitter:title" content="whatever title I have in data/ogp/twitter.yml" />

I want the Twitter description and title to be my overridden variable as well. This PR makes that possible.