Fix issue #164 for multiline string parsing

When parsing strings, the parser, on a newline, should look behind
for a backslash to determine if this is a multiline string.

The code below is an example of a valid CSS code that should be
compiled by scssphp without throwing an exception.

    body:after {
        content: "This is a \
    multiline string.";
    }
Fix typo in fix for #164
Add tests for #164