added ppm_writesession_t and redesigned ppm_write_header() and ppm_write_pixel() for this type. added example executable for writing ppm files.
added ppm_outstream_destroy_t callback type. added functions for creating a PPM Outstream from filename and fileobject. added pmm_outstream_destroy().
renamed ppm_writesession_init() to ppm_write_init()
added ppm_pixel_t and ppm_pixel_buffer_t
The main function now exports the game of life to a PPM image sequence.
implemented nicely aligned plain formatting.