Ignore bundle binstubs

Those who want to avoid typing `bundle exec` before each command that is
provided by a dependent gem, e.g. `yard`, `rspec`, `rubocop` itself,
Bundler provides a convenient way of creating binstubs in `bin`
directory. Commands can be run like `bin/yard`, `bin/rspec` etc then.

This can further simplify the usage by employing
[`direnv`](https://github.com/direnv/direnv) that has a nice option in
its `.envrc` file:

    PATH_add bin

After that (and approving `.envrc` changes with `direnv allow`) it's
possible to run `yard`, `rspec` and `rubocop` with no prefixes. I stick
to this in every repository I contribute to even if I have to deal with
unstaged changes in my `bin`.

Bundle doesn't generate those buildstubs accroding to best practices, so
we're better ignore them from internal investigations. Those are
machine-generated files for the machines.

.gitignore

@@ -62,3 +62,6 @@ TAGS
 
 # For rubinius:
 #*.rbc
+
+# For bundle binstubs
+bin/*

.rubocop.yml

@@ -13,6 +13,7 @@ AllCops:
     - 'spec/fixtures/**/*'
     - 'tmp/**/*'
     - '.git/**/*'
+    - 'bin/*'
   TargetRubyVersion: 2.4
 
 Naming/PredicateName: