Improve `livemark run`

[roll]

Overview

There are a lot of possible improvements we can add:

• visual indication of the names of executed runs
• a --show flag to show the run's code
• livemark data alias for livemark run data
• support a better UI for run scripts (show livemark run <run> on top of the code block)
• support livemark.yaml:task.sources == array to provide task sources not using pages
• rename livemark run to livemark task? (task runner) OR run=name to task=name?
• rebase from script to task as a snippet type?
• remove make dependency from livemark repo itself?
• rename livemark merge -> livemark update/etc?
• TBD

---

Using TaskPlugin:

'''python task id=data-extract
print('Data extract!')
'''