Add Regex for `Style/FileName` cop.

Library/.rubocop_rules.yml

@@ -98,10 +98,14 @@ Style/IfUnlessModifier:
   Enabled: false
 
 # dashes in filenames are typical
-# TODO: enforce when rubocop has fixed this
-# https://github.com/bbatsov/rubocop/issues/1545
 Style/FileName:
-  Enabled: false
+  # matches:
+  #   file_name.rb (default)
+  #   file-name.rb, --filename.rb (command names)
+  #   FILENAME.rb (ARGV and ENV)
+  # does not match:
+  #   dashes-and_underscores.rb
+  Regex: !ruby/regexp /^((([\dA-Z]+|[\da-z]+)(_([\dA-Z]+|[\da-z]+))*|(\-\-)?([\dA-Z]+|[\da-z]+)(-([\dA-Z]+|[\da-z]+))*))(\.rb)?$/
 
 # no percent word array, being friendly to non-ruby users
 # TODO: enforce when rubocop has fixed this

Library/Homebrew/cask/.rubocop.yml

@@ -91,7 +91,7 @@ Style/Documentation:
   Enabled: false
 
 Style/FileName:
-  Enabled: false
+  Regex: !ruby/regexp /^((([\dA-Z]+|[\da-z]+)(_([\dA-Z]+|[\da-z]+))*|(\-\-)?([\dA-Z]+|[\da-z]+)(-([\dA-Z]+|[\da-z]+))*))(\.rb)?$/
 
 Style/HashSyntax:
   EnforcedStyle: ruby19_no_mixed_keys