Fix #1: new <outputFormatter> option to change how task output is rendered

[rprieto]

Here's a PR that allows to configure the output prefix.

I initially added many options like prefixSymbol = '→', repeatPrefix = true etc... but in the end it seemed simpler to offer the option of a custom formatter. Let me know what you think.

Usage:

const list = new Listr(tasks, {
  renderer: ListrMultilineRenderer,
  outputFormatter: line => `→ ${line}`
});