Add `verbose` flag to show complete output

[andywer]

Two different use cases to consider:

"text/plain": Use different rendering if not in a TTY (if piping output into a log file, build server, ...)

• Don't show fancy output containing a lot of ANSI control characters, but rather simple plain text only
• See also: SamVerschueren/listr#11, SamVerschueren/listr#12
• Extending listr might be the best way to go

  "verbose": Show complete output of a task instantly (for debugging purposes)

• Might mean that tasks cannot be run in parallel

[andywer]

Blocked by SamVerschueren/listr#29, but apart from this bug it can now be done with new listr 0.6.0.