[FR] Clear an object from workspace

[azev77]

This applies more generally to @JuliaLang @JuliaComputing: other IDEs have a button to clear objects from the workspace it would be nice to do this for a single variable/all variables

[pfitzseb]

Julia doesn't really allow for this atm -- I think the best we can do is set the variable to nothing, which makes it gc'able. Getting rid of the binding itself isn't easy at all (we could kinda create a new Main module and copy all other bindings in the current scope to that, but I'd rather not get into that mess).