stb_image versioning

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#### Issue description



auto download of files won't work in packaging environment.



#### Your environment



 * version of mlpack:

 * operating system: OpenBSD

 * compiler:

 * version of dependencies (Boost/Armadillo):

 * any other environment information you think is relevant:



#### Steps to reproduce



since we have a jailed user for building packages, grabbing stb_image can't work



#### Expected behavior



I understand you don't want to include the files into mlpack proper because you want to be able to update independently, but please provide "versioned" urls to the files, so they can be grabbed AND checksummed correctly by maintainer.



instead of

http://mlpack.org/files/stb/stb_image.h



make it also available as something  like

http://mlpack.org/files/stb-2.22/stb_image.h



and

http://mlpack.org/files/stb/stb_image_write.h

available as

http://mlpack.org/files/stb-1.13/stb_image_write.h





That way, distribution vendors can grab the files and be sure they get a consistent

version that can be cryptographically checksummed.





#### Actual behavior



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