Libcaca environment variables

Some environment variables can be used to change the behaviour of libcaca without having to modify the program which uses it. These variables are:

- CACA_DRIVER: set the backend video driver. In order of preference:
  - `conio` uses the DOS conio.h interface.
  - `ncurses` uses the ncurses library.
  - `slang` uses the S-Lang library.
  - `x11` uses the native X11 driver.
  - `gl` uses freeglut and opengl libraries.
  - `raw` outputs to the standard output instead of rendering the canvas. This is can be used together with cacaserver.

- CACA_GEOMETRY: set the video display size. The format of this variable must be `XxY`, with `X` and `Y` being integer values. This option currently works with the raw, X11 and GL drivers.

- CACA_FONT: set the rendered font. The format of this variable is implementation dependent, but since it currently only works with the X11 driver, an X11 font name such as `fixed` or `5x7` is expected.