Task grouping

[kt3k]

This PR adds the function to group the tasks by the levels of headings, which has been discussed in #28.

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tasks

• [x] task_stack
• [x] task.children is TaskCollection
• [x] task.parent
• [x] parse hierarchy
• [x] update action info
• [x] testing parsing hierarchy
• [x] <!-- saku parallel --> <!-- saku parallel race -->
• [x] testing parallel in hierarchy
• [x] testing abort in hierarchy
• [x] Describe task grouping in README.md
• [x] refactor redundant things.

[codecov[bot]]

Codecov Report

Merging #31 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #31    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files          13     14     +1     
  Lines         298    398   +100     
======================================
+ Hits          298    398   +100

Impacted Files	Coverage Δ
pkg/saku/logger.go	100% <100%> (ø)	:arrow_up:
pkg/saku/task.go	100% <100%> (ø)	:arrow_up:
pkg/saku/action_run.go	100% <100%> (ø)	:arrow_up:
pkg/saku/run_options.go	100% <100%> (ø)	:arrow_up:
pkg/saku/run.go	100% <100%> (ø)	:arrow_up:
pkg/saku/action_info.go	100% <100%> (ø)	:arrow_up:
pkg/saku/action_version.go	100% <100%> (ø)	:arrow_up:
pkg/saku/task_stack.go	100% <100%> (ø)
pkg/saku/task_collection.go	100% <100%> (ø)	:arrow_up:
pkg/saku/action_help.go	100% <100%> (ø)	:arrow_up:
... and 2 more

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