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.
* Got rid of static buffers; we now use cucul_free() to free exported
buffers.
* Fixed light background in the ANSI exporter by adding escape sequences for
most terminal emulators.