Integrate blocking and multiplexing workers usage

features/cli/worker.feature

@@ -0,0 +1,9 @@
+Feature: worker CLI option
+
+  Scenario: uses the blocking worker when `block' is given
+    When I run uhwm with option -v -w block
+    Then the output must match /work.+event.+block/i
+
+  Scenario: uses the multiplexing worker when `mux' is given
+    When I run uhwm with option -v -w mux
+    Then the output must match /work.+event.+mux/i

features/steps/output_steps.rb

@@ -9,6 +9,7 @@ options:
     -f, --run-control PATH           specify alternate run control file
     -r, --require PATH               require ruby feature
     -l, --layout LAYOUT              specify layout
+    -w, --worker WORKER              specify worker
   eoh
 end