Commands from works when namespaced

A test for #201 and a more general fix.  Not sure if this is what I want to do
Unfortunately, `require` at any level of scope puts the code in global scope, meaning that if a GLI app wraps itself in a namespace, everything loaded via `commands_from` doesn't work.

The solution here is to grab the binding from `GLI::App` (where all the DSL commands are defined) and then `eval` the external commands.  Yikes, I know.