Fix multiple tasks of same type only handled once per snowblock config

When a task is defined multiple times within the same snowblock
configuration file, only the last object was processed while any object
before has been ignored.
The root cause was the `pkg/snowblock.TaskRunnerMapping` (1) custom type
that only accepted one `pkg/api.TaskConfiguration` (2) object.
Therefore any parsed task object of the same type was overriden (2) by
tasks that are parsed after that task resulted in missing tasks.

Before this commit, running this example configuration has not processed
the first `clean` task but only the second one:

```json
[
  {
    "clean": ["~/desktop/failure"]
  },
  {
    "link": {
      "~/desktop/success/config.json": {
        "create": true,
        "path": "config.json"
      }
    }
  },
  {
    "clean": ["~/desktop/success"]
  },
]
```

To fix the problem the `pkg/snowblock.TaskRunnerMapping` type now
accepts multiple `pkg/api.TaskConfiguration` (2) objects
(`TaskRunnerMapping map[api.TaskRunner][]api.TaskConfiguration`) instead
of only one so the previous object won't be overridden.

References:
  (1) https://github.com/arcticicestudio/snowsaw/blob/efdff96ec01f26bbf0a0d75bb9aab4cb86f023e8/pkg/snowblock/snowblock.go#L46
  (2) https://github.com/arcticicestudio/snowsaw/blob/988073b1bde8d7db4b40f259e99d218c959bba8f/pkg/api/snowblock/task.go#L18
  (3) https://github.com/arcticicestudio/snowsaw/blob/efdff96ec01f26bbf0a0d75bb9aab4cb86f023e8/pkg/snowblock/snowblock.go#L112

Epic: GH-33
Fixes GH-76

pkg/snowblock/snowblock.go

@@ -43,7 +43,7 @@ type Snowblock struct {
 	TaskObjects []api.Task
 
 	// TaskRunnerMapping contains the assignments from task objects to a matching task runner.
-	TaskRunnerMapping map[api.TaskRunner]api.TaskConfiguration
+	TaskRunnerMapping map[api.TaskRunner][]api.TaskConfiguration
 
 	// UnsupportedTasks is a list of task names that are not supported by an registered task runner.
 	UnsupportedTasks []api.TaskConfiguration
@@ -54,16 +54,18 @@ func NewSnowblock(path string) *Snowblock {
 	return &Snowblock{
 		Path:              path,
 		TaskObjects:       make([]api.Task, 0),
-		TaskRunnerMapping: make(map[api.TaskRunner]api.TaskConfiguration),
+		TaskRunnerMapping: make(map[api.TaskRunner][]api.TaskConfiguration),
 	}
 }
 
 // Dispatch handles the processing of the snowblock by dispatching the configured tasks to a registered runner that can
 // handle it.
 func (s *Snowblock) Dispatch() error {
 	for runner, instructions := range s.TaskRunnerMapping {
-		if err := runner.Run(instructions, s.Path); err != nil {
-			return err
+		for _, taskConfig := range instructions {
+			if err := runner.Run(taskConfig, s.Path); err != nil {
+				return err
+			}
 		}
 	}
 
@@ -109,7 +111,7 @@ func (s *Snowblock) Validate(taskRunner map[string]api.TaskRunner) error {
 		for taskName, taskConfigMap := range taskObject {
 			runner, exists := taskRunner[taskName]
 			if exists {
-				s.TaskRunnerMapping[runner] = taskConfigMap
+				s.TaskRunnerMapping[runner] = append(s.TaskRunnerMapping[runner], taskConfigMap)
 				continue
 			}
 			s.UnsupportedTasks = append(s.UnsupportedTasks, taskName)