Add support for setting variables

[draringi]

libucl has two functions for setting variable which are then used when parsing the input config files.
They are ucl_parser_register_variable and ucl_parser_set_filevars

[draringi]

In addition to ucl_parser_register_variable and ucl_parser_set_filevars, there exists ucl_parser_set_variables_handler, which allows the handling of unknown variables.
This would allow better integration with a programs internal data.