Allow conditions in variable node (#243)

* use condition in variable node

* added support for else expression

* addressing code review comments

CHANGELOG.md

@@ -22,7 +22,9 @@
 
 ### New Features
 
-_None_
+- Now you can conditionally render variables with `{{ variable if condition }}`, which is a shorthand for `{% if condition %}{{ variable }}{% endif %}`. You can also use `else` like `{{ variable1 if condition else variable2 }}`, which is a shorthand for `{% if condition %}{{ variable1 }}{% else %}{{ variable2 }}{% endif %}`  
+  [Ilya Puchka](https://github.com/ilyapuchka)
+  [#243](https://github.com/stencilproject/Stencil/pull/243)
 
 ### Internal Changes