Allow passing HOCON string via system property

[mkjinn]

References #380

Hello! This PR provides implementation of feature proposed by @havocp in thread #382

The goal is to have the ability to pass HOCON content as a string via system property config.config_content. Example: -Dconfig.config_content="param1=Hello\nparam2=world!".

This helps with some corner-case substitution scenarios, like passing empty list values via system props.

[lightbend-cla-validator]

Hi @mrmirg,

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

[ennru]

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