Add support for messageFormat strings containing conditionals

[timlohse1104]

See #441

It's now possible to pass --messageFormat option containing conditional statements for log data.

Example log data: {level: 30, req: {id: 'foo'}}

Example --messageFormat containing conditional: "{level} - {if req.id}({req.id}){end}{if msg} - {msg}{end}"

Example output: "30 - (foo)"

Unescaped if / end block within --messageFormat will just be deleted.

[coveralls]

Pull Request Test Coverage Report for Build 5649830685

• 14 of 14 (100.0%) changed or added relevant lines in 1 file 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 5620622122:	0.0%
Covered Lines:	419
Relevant Lines:	419

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💛 - Coveralls

[timlohse1104]

Updated documentation with --messageFormat condition example. Add matching for condition and property keys. Add further unit tests for parseMessageFormat function.

[timlohse1104]

Hey @jsumners thanks for the code review. I will look into it after work. Please excuse my poor english var and function wording. Will edit.

[jsumners]

Hey @jsumners thanks for the code review. I will look into it after work. Please excuse my poor english var and function wording. Will edit.

No problem. The review phase is there to get us into an agreeable state.