Add flag to emit multiline strings

[killercup]

based on #129 as it would otherwise conflict quite easily

[killercup]

See this as WIP for now -- there is only one (doc) test for it and I'm currently integration this into serde_yaml, and then use it in a project.

[karheinz]

@killercup I took your commit for emitting multiline strings (if enabled and appropriate) and applied it to master.

See merge request #137

[max-sixty]

I realize this is now very old, but would be great to have this functionality.

Maybe this could be a candidate for merging if someone forks the project...