allow for creating testdoubles using interface only

[lgandecki]

continuing from #399
@searls was right, the object function has all the necessary functionality built in. The only thing I had to tweak was this new type that allows for:

td.object<IMyInterface>(); 

but even with the current code testdouble actually allowed for mocking based on an interface, but with a bit of redundancy:

td.object<IMyInterface>("");

@wpannell you can use the second one already (with an argument of a string) for your training, with this merged in you will be able to remove the argument for a bit cleaner syntax.

[wpannell]

@lgandecki awesome

[searls]

Yep, seems legit. @lgandecki since I'm not really a TS user, would you mind opening a follow-on PR that does a couple things?

1. Documents this. I imagine a brief description in README and a docs/10-using-with-typescript.md file with examples or screenshots would really be great

2. Add typings to other top-level td APIs that might be better typified. Seems odd only td.object would do it.

[lgandecki]

Sure!
I'm off this week starting in a few hours, and I promised myself not to do anything programming-related... I will jump on it right away when I'm back.

Happy new year!

[searls]

Landed in 3.9.2

[lgandecki]

I've attempted something for 1. in #401 which to some degree also covers 2.
We could still make it so td.imitate<> works with its magic power, but it would require a little bit of hacking in the code itself, besides just changing the typings. I might give it a try at some point.