Add list command to CLI

This adds a list command, that shows all available
guard gems.

https://github.com/guard/guard/issues/111

lib/guard.rb

@@ -119,6 +119,16 @@ module Guard
       UI.error "Could not find 'guard-#{name}' gem path."
     end
 
+    ##
+    # Returns a list of guard Gem names installed locally.
+    def guard_gem_names
+      if Gem::Version.create(Gem::VERSION) >= Gem::Version.create('1.8.0')
+        Gem::Specification.find_all.select { |x| x.name =~ /^guard-/ }
+      else
+        Gem.source_index.find_name(/^guard-/)
+      end.map { |x| x.name.sub /^guard-/, '' }
+    end
+
     def debug_command_execution
       Kernel.send(:alias_method, :original_system, :system)
       Kernel.send(:define_method, :system) do |command, *args|

lib/guard/cli.rb

@@ -17,6 +17,27 @@ module Guard
       ::Guard.start(options)
     end
 
+    desc "list", "Lists guards that can be used with init"
+    def list
+      ::Guard::DslDescriber.evaluate_guardfile(options)
+      installed = []
+      ::Guard::DslDescriber.guardfile_structure.each do |group|
+        group[:guards].each {|x| installed << x[:name]} if group[:guards]
+      end
+
+      ::Guard::UI.info "Available guards:"
+      ::Guard::guard_gem_names.sort.each do |name|
+        if installed.include? name
+          ::Guard::UI.info "   #{name} *"
+        else
+          ::Guard::UI.info "   #{name}"
+        end
+      end
+      ::Guard::UI.info ' '
+      ::Guard::UI.info "See also https://github.com/guard/guard/wiki/List-of-available-Guards"
+      ::Guard::UI.info "* denotes ones already in your Guardfile"
+    end
+
     desc "version", "Prints Guard's version"
     def version
       ::Guard::UI.info "Guard version #{Guard::VERSION}"

spec/guard_spec.rb

@@ -130,6 +130,13 @@ describe Guard do
     end
   end
 
+  describe ".guard_gem_names" do
+    it "returns the list of guard gems" do
+      gems = Guard.guard_gem_names
+      gems.should include("rspec")
+    end
+  end
+
   describe ".supervised_task" do
     subject { ::Guard.setup }