Gets or sets various internal readline variables.
varnameA variable name.
newvalueIf provided, this will be the new value of the setting.
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 or two parameters, the old value is returned.