Add a default_logger

[ablaom]

A step towards resolution of #926; see also JuliaAI/MLJ.jl#1124.

This PR:

• Adds a global constant, initialized in __init__, for a default logger, for use with MLJFlow, for example.
• Adds a function default_logger for returning or resetting the default logger
• Changes the default value of the logger kwarg in evaluate, evaluate!, and Resampler from nothing to default_logger().
• Implements MLJBase.save(mach) = MLJBase.save(default_logger(), mach) to save machines (as artifacts) at the default logger (assuming it supports this).

To do:

• Add some tests

[ablaom]

@pebeto Any chance you could review this?