Add Jekyll::LiquidExtensions.lookup_variable

To use, just include `Jekyll::LiquidExtensions` as you please:

```ruby
class SayHi < Liquid::Tag
  include Jekyll::LiquidExtensions

  def initialize(tag_name, markup, tokens)
    @markup = markup.strip
  end

  def render(context)
    "hi #{lookup_variable(context, @markup)}"
  end
end
```

Fixes #2071.
Merge pull request #2253 from jekyll/var-substitution