CubeScript: fixes line continuation escape in strings

Summary:
Fix the line continuation escape in strings.

CubeScript files use the extension .cfg, which is used by default by Nagios, making it impossible to add a test file. It might be a good idea to add the extension *.cubescript and use it for the test file.

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D18948