Formatter makes a mess of strings joined with +

[dimbleby]

In:

module foo {
    namespace "http://example.com/foo/20181116";
    prefix "foo";

    leaf bar {
        type boolean;
        description
          "Lorem ipsum dolor sit amet, consectetur adipiscing elit, " +
          "sed do eiusmod tempor incididunt ut labore et dolore magna aliqua";
    }
}

out:

module foo {
    namespace "http://example.com/foo/20181116";
    prefix "foo";

    leaf bar {
        type boolean;
        description
          "Lorem ipsum dolor sit amet, consectetur adipiscing elit, " +
        + "sed do eiusmod tempor incididunt ut labore et dolore magna aliqua";
    }
}

We now have a + both at the end of line 8 and also at the start of line 9. That's one too many.