Add an API for disabling logging (T6955)

> Issue originally made by @maxbrunsfeld
### Bug information
- **Babel version:** v6.4.0
- **Node version:** v4.1.1
### Description

In the Atom editor, we use `babel-core` to compile ES6 files to ES5 on demand, at runtime. Windows users are reporting errors that happen when babel tries to [[ https://github.com/babel/babel/blob/master/packages/babel-core/src/transformation/file/logger.js#L50 | log to stderr ]], because windows GUI processes don't have a stderr stream.

It appears that the logging is [[ https://github.com/visionmedia/debug#usage | enabled or disabled by the `DEBUG` environment variable ]]. We would like to have some programmatic way of disabling the logging.