Hk cyclomatic complexity (#92)

* Proof of concept for cyclomatic complexity

* Enable configurability of rule

* Use the same nodes to increment the complexity as rubocop

* Fix typo in test description

* Properly indent code and simplify macro

* Move metric into metrics

* Cover a violation supressed by increased threshold

* Extract visitor into its own file

* Document cyclomatic complexity rule and visitor

* Refactor specs to use a macro

* Indent code inside macro

* Replace array with tuple for string formatting.

`Tuple` is stack based, whereas `Array` is allocated on the heap increasing GC pressure.

* Fix formatting

* Enable cyclomatic complexity rule by default

spec/ameba/ast/visitors/counting_visitor_spec.cr

@@ -0,0 +1,39 @@
+require "../../../spec_helper"
+
+module Ameba::AST
+  describe CountingVisitor do
+    describe "#visit" do
+      it "allow to visit ASTNode" do
+        node = Crystal::Parser.new("").parse
+        visitor = CountingVisitor.new node
+        node.accept visitor
+      end
+    end
+
+    describe "#count" do
+      it "is 1 for an empty method" do
+        node = Crystal::Parser.new("def hello; end").parse
+        visitor = CountingVisitor.new node
+
+        visitor.count.should eq 1
+      end
+
+      {% for pair in [
+                       {code: "if true; end", description: "conditional"},
+                       {code: "while true; end", description: "while loop"},
+                       {code: "until 1 < 2; end", description: "until loop"},
+                       {code: "begin; rescue; end", description: "rescue"},
+                       {code: "case 1 when 1; end", description: "when"},
+                       {code: "true || false", description: "or"},
+                       {code: "true && false", description: "and"},
+                     ] %}
+        it "increases count for every {{ pair[:description].id }}" do
+          node = Crystal::Parser.new("def hello; {{ pair[:code].id }} end").parse
+          visitor = CountingVisitor.new node
+
+          visitor.count.should eq 2
+        end
+      {% end %}
+    end
+  end
+end