Allow plugins to register custom static and lazy evaluated variables (Feature #790)

Currently, it is impossible for plugins to register custom variables to Liquid. This should be changed.

I propose the following API to register variables (Pull Request forthcoming):

 1ChiliProject::Liquid::Variables.register "macro_list",
 2  "Use '{{ variables | to_list: \"Variables\" }}' to see all Liquid variables and '{{ tags | to_list: \"Tags\" }}' to see all of the Liquid tags." 
 3
 4ChiliProject::Liquid::Variables.register "tags" do
 5  ::Liquid::Template.tags.keys.sort
 6end
 7
 8ChiliProject::Liquid::Variables.register "variables" do |context|
 9  vars = []
10
11  vars = context.environments.first.keys.reject do |var|
12    # internal variable
13    var == "text" 
14  end if context.environments.present?
15  vars += context.scopes.collect(&:keys).flatten
16  vars.uniq.sort
17end

The API will allow to register static variables (which contain data generated during loading of the Application) or lazily evaluated variables. These will be evaluated every time they are used.

The main difference to Drops is that these variables can be easily overwritten and act exactly the same as user-defined variables.