Render option to sort keys

[mmolimar]

[lightbend-cla-validator]

Hi @mmolimar,

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

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

[mmolimar]

Note: I have signed CLA.

[mmolimar]

The idea to implement this is to allow the API to sort the keys in the order you like setting your custom comparator. In my case, I have to sort these keys in a specific order so that the rendered output has the same output as a template I have expects. For example, in other libs such as spray-json you can set a custom printer to do this (among other things).

[ennru]

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