Add and apply Layout/DotPosition rule

Since IRB has improved its support for multiline, the main argument
towars using a trailing dot no longer affects most people.

It still affects me, though, since I use Pry :), but I agree
leading dots are more readable, so I'm enabling the rule anyway.

.rubocop.yml

@@ -21,6 +21,9 @@ Bundler/OrderedGems:
 Layout/AssignmentIndentation:
   Enabled: true
 
+Layout/DotPosition:
+  Enabled: true
+
 Layout/EmptyLineAfterGuardClause:
   Enabled: true