Added method to register boolean filters (#160)

* added method to register boolean filters

* parametrised negative filter name

* Update Extension.swift

* Update CHANGELOG.md

* renamed registerBooleanFilter to registerFilter

* updated docs

Sources/Extension.swift

@@ -18,6 +18,15 @@ open class Extension {
       return SimpleNode(token: token, handler: handler)
     })
   }
+  
+  /// Registers boolean filter with it's negative counterpart
+  public func registerFilter(name: String, negativeFilterName: String, filter: @escaping (Any?) throws -> Bool?) {
+    filters[name] = .simple(filter)
+    filters[negativeFilterName] = .simple {
+      guard let result = try filter($0) else { return nil }
+      return !result
+    }
+  }
 
   /// Registers a template filter with the given name
   public func registerFilter(_ name: String, filter: @escaping (Any?) throws -> Any?) {