Warn about unexpected multiline strings

[Sewi]

Add an a config option for this:

If you don't use multiline strings/regexps for yourself, they're usually an error.

There should be a config option to change the highlighter to show multiline strings (a ' or " or / starting on one line but ending on another one) in a warning color (like white-on-red-background).

I could add the config option if you do the highlighter changes :-)