Make formatting possible

[kastoestoramadus]

Allow the config library to be used as a formatter.

Rendering options added:

• don't reorder entries,
• control indentation,
• default assign operator,

#693

Let me know if such improvement will be accept to merge. For now I've seen your lib is in maintenance mode.

[lightbend-cla-validator]

Hi @kastoestoramadus,

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 Akka Contributors License Agreement:

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