AddConfigDocumentAPI

This work came out of the discussion in https://github.com/typesafehub/config/issues/272.

~~These commits add a new grouping of classes,

, to the library, which are used to form an AST. This AST contains all the tokens from which it was parsed, so the original text can be reproduced from them.~~

~~I'm opening this up for review now as the implementation is complete, so I think that portion is ready for review. The testing code is rough right now, largely because of the difficulty involved with crafting instances of

by hand, so I'm still working on improving that, although any feedback on that front is welcome.~~

This PR adds the ConfigDocument API in its entirety, including its backend representation with ConfigNodes, the ConfigDocument Parser, and the ConfigDocument Interface.

该提问来源于开源项目：lightbend/config