without using environment variables..
* Implement caca_get_display_driver() to return the current display's
output driver.
* Add a simple example program to test caca_create_display_with_driver().
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.
environment variables handling. Replaced that with three per-bitmap
functions: cucul_set_bitmap_antialias(), cucul_set_bitmap_color() and
cucul_set_bitmap_dithering().
* Added cucul_set_bitmap_brightness() and cucul_set_bitmap_contrast()
prototypes (but did not implement them yet).
* Added cucul_set_bitmap_charset() to choose what characters are going
to be used for the bitmap rendering.
* Removed the now useless cucul_get_feature(), cucul_set_feature() etc.