ENH: support full-scoping for keywords as lvalue (issue #429)

- patterns only supported for the final element.

  To create an element as a pattern instead of a word, an embedded
  string quote (single or double) is used for that element.
  Any of the following examples:

      "/top/sub/dict/'(p|U).*"     100;
      "/top/sub/dict/'(p|U).*'"    100;
      "/top/sub/dict/\"(p|U).*"    100;
      "/top/sub/dict/\"(p|U).*\""  100;

  are equivalent to the longer form:

      top
      {
          sub
          {
              dict
              {
                  "(p|U).*"     100;
              }
          }
      }

  It is not currently possible to auto-vivify intermediate
  dictionaries with patterns.

    NOK   "/nonexistent.*/value"   100;
    OK    "/existing.*/value"      100;

- full scoping also works for the #remove directive

        #remove "/dict1/subdict2/entry1"