Adding undocumented clean command to completion

Clean command is undocumented (not included in
bundle help output), however that is very useful,
especially in RVM environment you can clean up
outdated gems in gemset.

plugins/bundler/_bundler

@@ -23,6 +23,7 @@ case $state in
 			"viz[Generate a visual representation of your dependencies]" \
 			"init[Generate a simple Gemfile, placed in the current directory]" \
 			"gem[Create a simple gem, suitable for development with bundler]" \
+			"clean[Cleans up unused gems in your bundler directory]" \
 			"help[Describe available tasks or one specific task]"
 		ret=0
 		;;
@@ -62,6 +63,14 @@ case $state in
 			exec)
 				_normal && ret=0
 				;;
+			clean)
+				_arguments \
+					'(--force)--force[forces clean even if --path is not set]' \
+					'(--dry-run)--dry-run[only print out changes, do not actually clean gems]' \
+					'(--no-color)--no-color[Disable colorization in output]' \
+					'(--verbose)--verbose[Enable verbose output mode]'
+				ret=0
+				;;
 			(open|show)
 				_gems=( $(bundle show 2> /dev/null | sed -e '/^  \*/!d; s/^  \* \([^ ]*\) .*/\1/') )
 				if [[ $_gems != "" ]]; then