Easier tools for reading/setting/getting keys

Hi,
I'm interested in writing a program to update a config file, while preserving the comments in that file. This requires calling `ParseBytes()` or equivalent and then manipulating the resulting `*ast.File`. This is a little cumbersome. I added some helpers for getting/setting keys, in this gist:

https://gist.github.com/kevinburke/477ca3a49d67f886cd2ade96a2abaf2a

They assume the data has a little more structure than HCL provides - no listing two keys for a given ObjectList for example. But they significantly simplify the task of manipulating the AST, which is why I thought you might be interested in adding something like this to the project.

Start with ParseBytes() and the resulting *ast.File, then compare the proposed helpers in the linked gist with the AST structures exposed by HCL. Define how getting and setting keys should handle duplicate keys and other HCL cases; done means a reviewed helper API with coverage for the supported manipulations.

Written by the indexing model from the issue text.