Add Naming/Filename rule

spec/ameba/rule/naming/filename_spec.cr

@@ -0,0 +1,19 @@
+require "../../../spec_helper"
+
+module Ameba::Rule::Naming
+  subject = Filename.new
+
+  describe Filename do
+    it "passes if filename is correct" do
+      expect_no_issues subject, code: "", path: "src/foo.cr"
+      expect_no_issues subject, code: "", path: "src/foo_bar.cr"
+    end
+
+    it "fails if filename is wrong" do
+      expect_issue subject, <<-CRYSTAL, path: "src/fooBar.cr"
+
+        # ^{} error: Filename should be underscore-cased: foo_bar.cr, not fooBar.cr
+        CRYSTAL
+    end
+  end
+end