as bold or underline. No higher level support for these yet, but at
least everything is consistent.
* Created cucul_set_attr_ansi() and cucul_set_attr_argb() which are bound
to replace cucul_set_color() and cucul_set_truecolor().
such as cucul_dither_t instead of struct cucul_dither.
* Made cucul_buffer_t an opaque structure and implemented the two getters
cucul_get_buffer_data() and cucul_get_buffer_size().
* Documented all missing functions and function parameters.