v4: make userDataDir configurable

[DanielRuf]

Depending on the context in which the CLI is started, the path results in a different absolute path.

Please make this configurable to prevent, that it creates the tmp folder in the wrong place.

https://github.com/highcharts/node-export-server/blob/master/lib/browser.js#L69

[jszuminski]

Thanks for reporting @DanielRuf! I've added it to our backlog and will try to make it a part of the next minor release.

[jszuminski]

@DanielRuf feel free to test out this pull request and let me know if that's what you were looking for.