If a command does `fmt.Print(...)` for output that contains ANSI color
codes, this not safe on Windows. We have to ensure that we always use
the `fmt.Fprint*` family of functions with a writer that was transformed
using `utils.NewColorable()`.
This makes the approach from `pr list` reusable across other commands
that may benefit from table-based output, e.g. `issue list` or `pr status`
The idea is: instantiate a printer, connect it to stdout, feed it some
data, and it does the rest: colored, truncated column output that fits
into a terminal, or tab-delimited output (no color, no truncation) for
scripts.
