Be explicit that strings cannot span across multiple lines

I mean, they have to be single line anyway.

Rationale:
- There is a correct way to encode line separators with special characters.
- Multi-line strings would be confusing in arrays anyway.
- Windows again (see #113): forcing strings to be single line helps when a user wrongly specifies a stranded backslash at the end of the string. That way it won't break parsing the rest of the configuration file.