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.
* 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.