If called with no parameters, this function returns an
array of values for all the setting readline uses. The
elements will be indexed by the following values: done, end,
erase_empty_line, library_version, line_buffer, mark,
pending_input, point, prompt, readline_name, and
terminal_name.
If called with one parameter, the value of that setting
is returned. If called with two parameters, the setting will
be changed to the given value.