Implement a robust parser PPM/PGM file header

[gonzalezsieira]

From http://paulbourke.net/dataformats/ppm/:

The following are all valid PPM headers.

Header example 1

P6 1024 788 255

Header example 2

P6 
1024 788 
# A comment
255

Header example 3

P3
1024 # the image width
788 # the image height
# A comment
1023

Currently, only type 2 is supported.