add utility to compose paths

[rodneyrehm]

URI.js is currently lacking a utility like Node's path.join() to simply concatenate a few tokens to generate a path. Here's a naive (and not quite accurate) approach:

URI.joinPaths = function() {
  var segments = [].reduce.call(arguments, function(previous, current) {
    var _segments = new URI(current).segment();
    return previous.concat(_segments);
  }, []).filter(Boolean);

  return new URI('').segment(segments);
};

URI.joinPaths('/a/b', '/c', 'd', '/e').toString();
// "a/b/c/d/e"
// (should have leading "/")

URI.joinPaths('a/b', 'http://example.com/c', new URI('d/'), '/e').toString();
// "a/b/c/d/e"

URI.joinPaths('a').toString();
// "a"

URI.joinPaths('').toString();
// "/"
// (should be empty string "")

URI.joinPaths('a', '', '', 'b', '', '').toString();
// "a/b"
// (should have trailing "/")

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Somewhat related is the need to resolve paths similarly to Node's path.resolve() but that should probably be a separate PR.