Feature request: Allow optional buffer(s) instead of `src` file/folder

[davej]

I already have the buffers of the source files in memory. It would be nice if I could just pipe the buffers straight to icon-gen. Right now I need to temporarily write the files to disk before icon-gen can use them.

[mifi]

or streams would be even more flexible, e.g. createReadStream or Readable.from(myBuffer). should be pretty simple to implement with sharp