Add timestamp to task output

[ivotron]

Add an option that instructs Task to print a timestamp when a task is started. Something like:

[2020-04-08 10:53:53] task: [my-task] ...

Not sure if this option would be enabled with a flag or YAML item. The format of the timestamp could also be customized.

This was briefly mentioned in https://github.com/go-task/task/pull/398#issuecomment-769331805

[ivotron]

the other alternative that occurs to me is to add a "report" at the end of the execution that shows a breakdown of how long each task took to run. I think that'd actually be more desirable in my case. The scenario we're having is that we run several tasks in parallel and it's hard to know how long each one is taking due to interleaved output and lack of proper timestamps from the software being invoked