Allow to specify image format when rendering to file

[alno]

According to native save_to_file function - second argument is optional format name.

For example, with this patch 256-color png may be forced by passing 'png256' (by default .png rendered as 24-bit)

[springmeyer]

Thanks! Keep em coming :)

I'll let @elliotlaster take a look at this one and merge it, since I'm not a ruby expert.

[elliotlaster]

Thanks so much!