Tasks

Add ability to specify custom tasks that could be invoked in hooks:

```
# Define tasks in shuttle config
tasks:
  my_task: "do something"
  another_task:
    - "cmd1"
    - "cmd2"

# Trigger tasks in hooks
hooks:
  before_link_release:
    - "task: my_task"
    - "task: another_task"
    - "something else"
```