Disallows while true (#22)

* Disallows `while true`

* Add syntax highlighting in documentation

* Replace `while true` occurrencies

* Add WhileTrue rule to config sample

spec/ameba/rule/while_true_spec.cr

@@ -0,0 +1,42 @@
+require "../../spec_helper"
+
+valid_source = <<-EOF
+a = 1
+loop do
+  a += 1
+  break if a > 5
+end
+EOF
+
+invalid_source = <<-EOF
+a = 1
+while true
+  a += 1
+  break if a > 5
+end
+EOF
+
+module Ameba::Rule
+  subject = WhileTrue.new
+
+  describe WhileTrue do
+    it "passes if there is no `while true`" do
+      source = Source.new valid_source
+      subject.catch(source).should be_valid
+    end
+
+    it "fails if there is `while true`" do
+      source = Source.new invalid_source
+      subject.catch(source).should_not be_valid
+    end
+
+    it "reports rule, pos and message" do
+      source = Source.new invalid_source, "source.cr"
+      subject.catch(source).should_not be_valid
+
+      error = source.errors.first
+      error.location.to_s.should eq "source.cr:2:1"
+      error.message.should eq "While statement using true literal as condition"
+    end
+  end
+end