Tuning using RandomSearch #86

Be good to add this. Very basic example is in the manual:

https://alan-turing-institute.github.io/MLJ.jl/dev/tuning_models/#Tuning-using-a-random-search-1

which also includes the detailed doc-string. The doc-string for StatsBase.fit there also has relevant information about how various distributions get fit to a range if an explicit prior is not specified.