Support default values for variables

[avichalp]

Support default values in variable definitions, such as $owner, $repo in the following query.
Here is the graphql spec for defaults.

[:query
   "MyQuery"
   {:$owner 'String! :$repo 'String!}
   [[:repository
     "repo"
     {:owner '$owner, :name '$repo}
     [[:issues
       "closedIssues"
       {:last 20, :states 'CLOSED}
       [[:edges
         [[:node
           [[:title]
            [:url "picture"]
            [:labels
             {:first 5}
             [[:edges
               [[:node
                 [[:name]]]]]]]]]]]]]]]]]