Updated config file location handling to be os agnostic

[MichaelOrtho]

If this plugin is run on Linux (mono), it breaks because of two reasons:

1. NewRelic.Platform.Sdk newrelic.json file path handling
   (Updated config (newrelic.json) handling, resolves #41 newrelic_dotnet_sdk#42)
2. Updated file handling plugin.json inside this code change

QUESTION: To wait for a release of SDK or to go forward with this one and to create another pull request when SDK change passes?

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