Support labels for commands

[Chi-teck]

Motivation

When running multiple commands it sometimes difficult to distinguish their ouput.

cmds:
  - some/script.sh
  - task: some_task
  - another/script.sh
  - task: another_task

Proposed solution

Add label property to cmd difinition so the above example could be modified as follows.

cmds:
  - label: Step 1
    cmd: ./some/script.sh
  - label: Step 2
    task: some_task
  - label: Step 3
    cmd: another/script.sh
  - label: Step 4
    task: another_task

Currently it requires adding extra commands like follows

cmds:
  - echo -e "\e[0;32mStep 1\e[0m"
  - some/script.sh

This doesn't look well. Also the color scheme may not be consistent with the colors used by Task itself.