Change -W command to accept a single file

Currently setting workflows with the -W (--workflows) flag only accepts
a folder. It is not possible to run a single workflow.yml file.

This commit allows either a single file or a directory to be passed into
the workflows flag.

cmd/input.go

@@ -51,7 +51,7 @@ func (i *Input) Workdir() string {
 	return i.resolve(".")
 }
 
-// WorkflowsPath returns path to workflows
+// WorkflowsPath returns path to workflow file(s)
 func (i *Input) WorkflowsPath() string {
 	return i.resolve(i.workflowsPath)
 }