Add SkipFields option

[gillepsi]

Adds a SkipFields option which omits the given fields from standard output. This is useful if you have a CloudWatch hook and want certain fields to be included in the CloudWatch logs, but not in the standard output.

For my circumstance, I omitted correlation ID equivalent fields, which are useful in logs, but is excessive detail in the standard output.

[coveralls]

Pull Request Test Coverage Report for Build 50

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 0.0%

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Totals
Change from base Build 48:	0.0%
Covered Lines:	0
Relevant Lines:	0

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💛 - Coveralls

[dfateyev]

By the way, I have recently stumbled upon a similar issue. But, I found the approach of deleting message fields at the message formatting stage incorrect by design for a generic message formatter.
A specialized formatter in Hook with a limited and tailored functionality will do things faster and cleaner than the generic one with many options to consider.

Also, sometimes it's possible to use a more suitable logging context, without fields to skip.

@antonfisher IMO, we can close this PR without merging.