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.