Expose the AST/CST for interation. Fixes #300

[patrickptibco]

This exposes an api for AST/CST interaction. I followed the general plan of attack as outlined in #300. Fixes #300

With these changes you can now edit HOCON files and preserve whitespace, comments, and variable references.

[lightbend-cla-validator]

Hi @patrickptibco,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Lightbend Contributors License Agreement:

https://www.lightbend.com/contribute/cla

[patrickptibco]

I have signed the CLA

[lightbend-cla-validator]

Hi @patrickptibco,

Thank you for your contribution! We really value the time you've taken to put this together.

We see that you have signed the Lightbend Contributors License Agreement before, however, the CLA has changed since you last signed it.
Please review the new CLA and sign it before we proceed with reviewing this pull request:

https://www.lightbend.com/contribute/cla

[patrickptibco]

Grr... I'd love to avoid dealing with my legal department again, what are the changes in the new CLA?

The changes since last time are just bumping down the required java version as toUnmodifiableList was 11 only, and fixing two missed tests, along with a merging of master.

[ennru]

We do not intend to extend the functionality of "Typesafe Config" further.
See https://github.com/lightbend/config#maintained-by