Added support for brackets in boolean expressions (#165)

* added support for brackets in boolean expressions

* more descriptive error messages

* use array slices

* added test for nested expressions

* removed brackets validation step

* address code review comments

* added doc comment

* simplify expression spec

* fixed docs

CHANGELOG.md

@@ -16,6 +16,9 @@
 - Now requires Swift 4.1 or newer.  
   [Yonas Kolb](https://github.com/yonaskolb)
   [#228](https://github.com/stencilproject/Stencil/pull/228)
+- You can now use parentheses in boolean expressions to change operator precedence.  
+  [Ilya Puchka](https://github.com/ilyapuchka)
+  [#165](https://github.com/stencilproject/Stencil/pull/165)
 
 ### New Features