Make @edit work with packages

[davidanthoff]

It would be great if @edit MyPackage would open an editor and pass the folder name where MyPackage is located (according to the current env) as an argument.

[Keno]

Should probably throw an error if the package isn't dev'ed, since modifying packages in situ isn't otherwise supported.

[cossio]

Since @edit even works to open functions in Base, which should not be modified either, I think an error would not be consistent.

I set my JULIA_EDITOR to read only vim, vim -R. This way @edit is safe for me.