Configure Simplecov in .simplecov

Externalizing the config into `.simplecov` is considered best practice:
simplecov-ruby/simplecov#235 (comment)

.simplecov

@@ -0,0 +1,35 @@
+# https://github.com/colszowka/simplecov#using-simplecov-for-centralized-config
+# Maybe put some conditional here not to execute the code below unless ENV['COVERAGE'] == 'true'
+# Lookup defaults: https://github.com/colszowka/simplecov/blob/master/lib/simplecov/defaults.rb
+SimpleCov.start do
+  load_profile 'test_frameworks'
+  coverage_dir 'coverage'
+  command_name 'MiniTest'
+  merge_timeout 3600 # 1 hour
+  track_files "{app,lib}/**/*.rb"
+
+  # Groups
+  add_group 'Controllers', 'app/controllers'
+  add_group 'Models', 'app/models'
+  # add_group 'Mailers', 'app/mailers'
+  add_group 'Helpers', 'app/helpers'
+  add_group 'Jobs', %w(app/jobs app/workers)
+  add_group 'Libraries', 'lib'
+
+  add_group 'Long files' do |src_file|
+    src_file.lines.count > 100
+  end
+  class MaxLinesFilter < SimpleCov::Filter
+    def matches?(source_file)
+      source_file.lines.count < filter_argument
+    end
+  end
+  add_group 'Short files', MaxLinesFilter.new(5)
+
+  # Exclude these paths from analysis
+  add_filter '/config/'
+  add_filter '/db/'
+  add_filter 'lib/plugins'
+  add_filter 'vendor'
+  add_filter 'bundle'
+end

spec/spec_helper.rb

@@ -1,3 +1,4 @@
+require 'simplecov'
 # This file was generated by the `rails generate rspec:install` command. Conventionally, all
 # specs live under a `spec` directory, which RSpec adds to the `$LOAD_PATH`.
 # The generated `.rspec` file contains `--require spec_helper` which will cause