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

src/ameba/rule/naming/filename.cr

@@ -0,0 +1,28 @@
+module Ameba::Rule::Naming
+  # A rule that enforces file names to be in underscored case.
+  #
+  # YAML configuration example:
+  #
+  # ```
+  # Naming/Filename:
+  #   Enabled: true
+  # ```
+  class Filename < Base
+    properties do
+      description "Enforces file names to be in underscored case"
+    end
+
+    MSG = "Filename should be underscore-cased: %s, not %s"
+
+    private LOCATION = {1, 1}
+
+    def test(source : Source)
+      path = Path[source.path]
+      name = path.basename
+
+      return if (expected = name.underscore) == name
+
+      issue_for LOCATION, MSG % {expected, name}
+    end
+  end
+end