Add `toAnsiString` to Canvas and Image when compiled with libcaca

This is almost a joke, but someone might find it useful for print-debugging (especially if they are developing on a remote server).

I used some code from img2txt in [libcaca](http://caca.zoy.org/wiki/libcaca), which should't be a problem according to it's [Do What The Fuck You Want To Public License](http://www.wtfpl.net/faq/).