Implemented dry run mode

Added a --dry-run flag that compiles and steps through each task, but does not execute them. The commands that would have been run are printed. See #125.

task.go

@@ -35,6 +35,7 @@ type Executor struct {
 	Watch    bool
 	Verbose  bool
 	Silent   bool
+	DryRun   bool
 
 	Context context.Context
 
@@ -211,6 +212,10 @@ func (e *Executor) runCommand(ctx context.Context, t *taskfile.Task, call taskfi
 			e.Logger.Errf(cmd.Cmd)
 		}
 
+		if e.DryRun {
+			return nil
+		}
+
 		stdOut := e.Output.WrapWriter(e.Stdout, t.Prefix)
 		stdErr := e.Output.WrapWriter(e.Stderr, t.Prefix)
 		defer stdOut.Close()