title: Guard: cmd option doesn't seem to be evaluated
body: <p>I try to get Guard running with the Spring gem.</p>
  
  <p>Spring works nicely on the console, and I want Guard to use <code>cmd: 'spring rspec'</code>, but Guard doesn't seem to care about the <code>cmd:</code> parameter ind the <code>Guardfile</code>:</p>
  
  <pre><code>guard :rspec, cmd: 'blaaaa' do
  </code></pre>
  
  <p>This doesn't result in an error, so I think it's simply omitted. How can I debug this?</p>
  
  <p>Gemfile:</p>
  
  <pre><code>group :development do
    gem 'spring'
    gem 'spring-commands-rspec' # Commands for RSpec
    gem 'listen', '~&gt; 1.0'
  
    gem 'guard-rspec', require: false # Automatically run tests
  end
  </code></pre>
  
  <p>Guardfile:</p>
  
  <pre><code># A sample Guardfile
  # More info at https://github.com/guard/guard#readme
  
  guard :rspec, cmd: 'spring rspec' do
    watch(%r{^spec/.+_spec\.rb$})
    watch(%r{^lib/(.+)\.rb$})     { |m| "spec/lib/#{m[1]}_spec.rb" }
    watch('spec/spec_helper.rb')  { "spec" }
  
    # Rails example
    watch(%r{^app/(.+)\.rb$})                           { |m| "spec/#{m[1]}_spec.rb" }
    watch(%r{^app/(.*)(\.erb|\.haml|\.slim)$})          { |m| "spec/#{m[1]}#{m[2]}_spec.rb" }
    watch(%r{^app/controllers/(.+)_(controller)\.rb$})  { |m| ["spec/routing/#{m[1]}_routing_spec.rb", "spec/#{m[2]}s/#{m[1]}_#{m[2]}_spec.rb", "spec/acceptance/#{m[1]}_spec.rb"] }
    watch(%r{^spec/support/(.+)\.rb$})                  { "spec" }
    watch('config/routes.rb')                           { "spec/routing" }
    watch('app/controllers/application_controller.rb')  { "spec/controllers" }
  
    # Turnip features and steps
    watch(%r{^spec/acceptance/(.+)\.feature$})
    watch(%r{^spec/acceptance/steps/(.+)_steps\.rb$})   { |m| Dir[File.join("**/#{m[1]}.feature")][0] || 'spec/acceptance' }
  end
  </code></pre>