Introduce programTransformers option

[saitonakamura]

Here's the story, I have a transformer that can generate a runtime validation for json based on a generic type parameter. For that I need to have an access to a type checker, but vanilla transformers can't do that. I came across a solution in ts-loader: instead of passing an object literal, you pass a function that accepts a Program and should return that object with transformers. So I implemented the same api in ts-node considering the backwards compatibility (it's a brand new separate option). While implementing I came across chicken-egg problem cause LanguageServiceHost accepts transformers function, but you can only get an access to program once you create a LanguageService itself, so i'm kinda using the variable that will be initialized later. It's working, though it can be fragile with different typescript versions.

I'm also not sure about the name of the option programTransformers. It doens't sound like a perfect one, but i'm yet to find out the better alternative. @blakeembrey maybe you have some ideas?

Fixes #792

[coveralls]

Coverage decreased (-1.2%) to 84.146% when pulling 616c481 on saitonakamura:feat/program-transformers into 6295254 on TypeStrong:master.

[saitonakamura]

@blakeembrey sorry for bugging, what are your thoughts on this?

[blakeembrey]

@saitonakamura This looks great, but how about we just make it a union signature of Transformers | (program) => Transformers? That's also backward compatible.

[saitonakamura]

@blakeembrey yeah, totally, I don't know why I haven't thought about it in the first place, too much C# I guess 😃 . Thank you, I've done it and it helped to simplify the code!

[saitonakamura]

Thanks for merging and refactoring it!