feat: allow path renaming

[mvegter]

This can allow users to remove the .nyc_output from the filePath, that way coverage tools such as Codecov can actually report coverage.

[coveralls]

Pull Request Test Coverage Report for Build 168

• 10 of 10 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 99.645%

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Totals
Change from base Build 165:	0.01%
Covered Lines:	229
Relevant Lines:	229

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💛 - Coveralls

[dword-design]

@mvegter I think v8-to-istanbul needs the storage of the generated files, but I made a PR that still maps the paths back to their originals at the very end #64.