Allow specifying custom storage directory

[AlexBulankou]

After the latest changes, offline storage location is os.tmpdir()/appInsights-node+<ikey>. We should add an option for a user to customize it so they can write it to a custom ACL-protected directory to prevent other processes that have access to tmpdir to read it.

[OsvaldoRosado]

Update: Part of this reasoning is no longer necessary (SDK will ACL protect the directory made in the temp folder), but we should still allow overriding the directory anyway.