Adds global `beforeEach` and `afterEach` hooks.

Adds global hooks to run code before and after each test suite (file).
These are specified in the file loaded using the `globals_path`
setting in `nightwatch.json`. They may be synchronous or asynchronous.
Merge branch 'global_before_each_hooks' of https://github.com/mildmojo/nightwatch into mildmojo-global_before_each_hooks
skipping global beforeEach/afterEach in case the module is disabled
Merge branch 'mildmojo-global_before_each_hooks'