Provide a way to get task completion progress

[dantibel]

There should be a method to get % value of task completion
(Original issue is 4J-company/mr-importer#2)

[cone-forest]

This would imply adding some float progress; to the mr::TaskBase class. Then after each stage completes, it updates its progress value, which then is used to compute progress of the parent stage.
To achieve better accuracy it's better to calculate progress on every call of progress() getter