Multitask option to set the default behavior when a subtask is not specified

Sometimes it's dangerous or undesired to run all of a multitask's subtasks by default.

Proposal: an option to set the default behavior when a subtask is not specified instead of iterating through all the subtasks.

Default `deploy` to use `dev` when a subtask is not specified:

```
deploy: {
options: {
default: 'dev'
},
dev: { .. },
qa: { .. },
prod: { .. }
}
```

```
$ grunt deploy # only runs deploy:dev instead of all of them
```

Example:
Don't run any bump subtasks if the subtask is not specified.

```
bump: {
options: {
default: false
},
major: { .. },
minor: { .. },
patch: { .. },
build: { .. }
}
```

```
$ grunt bump # returns an error that the bump subtask was not specified
```

No source file, test, or entry point is named. Start by tracing how Grunt resolves multitask invocations and handles omitted subtasks, then review existing multitask tests. Done means configurations can select a default subtask or explicitly reject an omitted subtask without changing existing behavior unexpectedly.

Written by the indexing model from the issue text.