Make artifact retrieval more convenient

[ablaom]

I wonder while we think of a remedy for #42, we can think about a better way for MLJ users to retrieve artifacts, without needing familiarity with the MLFlowClient API, and so requiring so many steps. What about something along the lines of a function MLJFlow.artifact(logger, run) to enable this workflow:

run = MLJ.save(logger, mach)
restored_mach = MLJFlow.artifact(logger, run)

Also, what if the user failed to record run? Is there no way to retrieve the artifact? Can artifacts be automatically tagged in some way that provides a user-handle for later retrieval, without knowing actual run object? So, instead of run above, we can subsitute the string handle?