feat: add shield:model command

[paulbalandan]

Supersedes and closes#491

[MGatner]

Renaming it MyUserModel means that factories won't pick it up automatically when searching for UserModel. This seems a loss to me, maybe not for the library itself but for any supporting code.

[kenjis]

UserModel instead of MyUserModel seems better for the default sample.
There is no need to prefix My.

[kenjis]

Oh, I get an error if I specify UserModel. It's a bug.

[lonnieezell]

When I tried with UserModel as the generated filename, I think the only issue was the class name conflict. I've made suggestions here to fix that in the code (though not the docs).

I think this will fix @MGatner concerns, which I agree with.

Other than that, my only suggestion might be to provide a default classname at the cli prompt.

[MGatner]

I haven't run this but the code looks good. Thanks for making that change.

[lonnieezell]

Looks great. Thanks for taking this one over.

[datamweb]

It's a bug.

@kenjis waiting for you. Did your worries go away?

[kenjis]

LGTM!

[datamweb]

Thanks all.