generate unique runtime configs per webdriverio execution

[zinserjan]

Webdriverio's test runner does not provide any api to pass data from launcher to runner processes for custom services. #66 used a workaround to pass the data to the child process with a temporary file like suggested in webdriverio/webdriverio#1914. But the name of the file was fixed and leads to wrong results when running multiple instances of webdriverio at the same time.

This PR improves the workaround and avoids conflicts when you run several instances of webdriverio simultaneously.

Running simultaneously the same config file requires a Node version with at least 6.13.0, 8.10.0, 9.2.0. In previous versions process.ppid isn't available. But this should be a edge case (maybe a wdio config that is configured via environment variables?).

Changes:

• move runtime config logic to BaseCompare -> every plugin can use this
• further calls to read the config are cached
• build a "unique" filename per webdriverio execution on the basis of the provided cli arguments and when available the process id.