* Create stupidly simple retrieval functions, as such:

    xml_value_get_string(val,
                         key,
                         // output
                         &s);

  which will:

    * ignore errors in key
    * if a success match is found, then s is freed if non-zero,
      and the new value is assigned to s.

* Similarly

    xml_value_get_int(val,
                      key,
                      /* output */
                      &s);
    xml_value_get_double(val,
                    	  key,
                          /* output */
                          &s);
    xml_value_get_bool(val,
                       key,
                       /* output */
                       &s);
  These three will override the value in s iff the key is succesfully
  matched.

* The error version will be available only for value version.

* Rewrite the parser to make it thread safe.

* Introduce variable interpolation inside double quotes.

* Support binary chunk values by the following syntax:

    <size=345667>...345667 bytes follow...