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/rule/metrics/cyclomatic_complexity_spec.cr

@@ -0,0 +1,47 @@
+require "../../../spec_helper"
+
+module Ameba::Rule::Metrics
+  subject = CyclomaticComplexity.new
+  complex_method = <<-CODE
+    def hello(a, b, c)
+      if a && b && c
+        begin
+          while true
+            return if false && b
+          end
+          ""
+        rescue
+          ""
+        end
+      end
+    end
+  CODE
+
+  describe CyclomaticComplexity do
+    it "passes for empty methods" do
+      source = Source.new %(
+        def hello
+        end
+      )
+      subject.catch(source).should be_valid
+    end
+
+    it "reports one issue for a complex method" do
+      subject.max_complexity = 5
+      source = Source.new(complex_method, "source.cr")
+      subject.catch(source).should_not be_valid
+
+      issue = source.issues.first
+      issue.rule.should eq subject
+      issue.location.to_s.should eq "source.cr:1:1"
+      issue.end_location.to_s.should eq "source.cr:12:3"
+      issue.message.should eq "Cyclomatic complexity too high [8/5]"
+    end
+
+    it "doesn't report an issue for an increased threshold" do
+      subject.max_complexity = 100
+      source = Source.new(complex_method, "source.cr")
+      subject.catch(source).should be_valid
+    end
+  end
+end