Allow unquoted strings with underscores

[kmecpp]

Might be a long shot, but I don't believe this modifies the spec and it would be nice for those who prefer underscores over hyphens

[lightbend-cla-validator]

Hi @kmecpp,

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:

http://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