Detailed global enable

[aswanson08]

Modified the global enable to allow a finer degree of control if needed. This branch allows global enabling/disabling of adds, deletes, or modifications individually. For example, if audit logs are only needed when a record is modified or deleted, but there is no need to log when a record is added, simply add the following line to your setup:

GlobalTrackingConfig.AdditionsEnabled = false;

You can also do this through any instance of ITrackerContext as well, as shown:

trackerContext.AdditionTrackingEnabled = false;

The original Enabled property on the GlobalTrackingConfig and the TrackingEnabled property on the TrackerContext can be used to enable/disable all three types of logs at once, and when read will tell you if at least one of the three types of logs is enabled.

[asulwer]

I am manually merging this into the EF Core 2 branch I have been working on