Add new option to fallback custom levels and colors to default values

[bitDaft]

Currently the customLevels and customColors options completely override the default Levels.

This PR changes the default behaviour of either using customLevels or default levels, and instead makes it so that it will now override the default levels with new Levels.

The reason for this change is that there is a use case where we may only want to add a few extra custom levels and need to use the existing levels too.

refering to comment in issue #62

[mcollina]

I don't think we can accept this breaking change. Could you add new options instead?

[coveralls]

Pull Request Test Coverage Report for Build 2009223414

• 0 of 9 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 1997621969:	0.0%
Covered Lines:	364
Relevant Lines:	364

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 2011357941

• 44 of 44 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 1997621969:	0.0%
Covered Lines:	380
Relevant Lines:	380

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💛 - Coveralls

[bitDaft]

Refactored to maintain backward compatibility.

Now uses a new option to switch whether to fallback level and color option to default values

The option name is useOnlyCustomProps, it's default value is true; to get the fallback, false can be passed as value.

The default is kept as true, to keep inline with how pino uses the useOnlyCustomLevels option which needs to be true to override the levels. Maybe this option can be set to default false in the next major version to follow along with pino convention of overriding and adding custom levels

[mcollina]

Good work! Could you add the new option to the README and update the PR title?

[bitDaft]

updated

[mcollina]

lgtm

[bitDaft]

@mcollina can this PR be merged?