omit page content front created context

remove page content properties (`pages` and `pagination`) from `this` and `opts` contexts before creating final context.  this is done to prevent LoDash templates in content being unintentionally evaluated and failing due to missing properties. see https://github.com/helpers/handlebars-helper-partial/issues/3

currently there is no test for this condition since i am not sure how the tests work yet ;)

note that this fix should eventually end up in https://github.com/helpers/handlebars-helper-include as well or better yet they should share code base or lib.