feat(log): provide custom formatting for every entry kind

[antongolub]

continues #1122

$.log.formatters = {
  cmd: (entry: LogEntry) => `CMD: ${entry.cmd}`,
  fetch: (entry: LogEntry) => `FETCH: ${entry.url}`
}

• Tests pass
• Appropriate changes to README are included in PR

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[antonmedv]

Do we really need a separate formatter, and a separate API for formatter for fetch? We kinda own all fetch functionality and can format as we'd like to.

[antongolub]

We capture the init opts which may contain headers with creds. The best place to attach masker if necessary w/o overriding the whole log.