Expose logging configurations to users

Currently Doltpy implements logging in a suboptimal way: each file has a logging object with it's name passed. This means the library does not have an ability to configure the log level without importing the `logger` and then manually setting the log level (it's not _that_ bad, but it requires knowing about the variable, and it's not documented).

Instead, we should implement a global log level variable for `doltpy.core` and expose it in the user facing API with a sensible default. Loggers should then be instantiated from a shared factory method that uses this shared configuration. We can also use this to enable library users to specify handlers etc.