No way to override log file path

[carestad]

In https://github.com/browserstack/browserstack-local-php/blob/master/lib/Local.php#L17 there is a hardcoded link to where local.log is being stored, which can not be overridden. This file usually ends up in my project's root folder to great annoyance.

Could this for instance be fetched from the environment like the BROWSERSTACK_ACCESS_KEY, but default to getcwd() . "/local.log" it that is not set?

Would you accept a pull request on the matter?