(maint) Add support for Gemfile.local

This commit adds support for having a Gemfile.local next to the Gemfile
provided by the project that can supply additional gems, such as debuggers.

Author: Sharpie

Diff for: .gitignore

@@ -2,6 +2,7 @@ vendor/*
 output/*
 .bundle
 gems
+Gemfile.local
 Gemfile.lock
 *.log
 ext/packaging

Diff for: Gemfile

@@ -16,3 +16,5 @@ gem 'packaging', *location_for(ENV['PACKAGING_LOCATION'] || '~> 0.99.8')
 gem 'rake', '~> 12.0'
 
 #gem 'rubocop', "~> 0.34.2"
+
+eval_gemfile("#{__FILE__}.local") if File.exist?("#{__FILE__}.local")