Support TIFF images.

[mikecx]

fixes #107

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[mikecx]

I signed it!

[googlebot]

CLAs look good, thanks!

[willnorris]

why can't we encode as tiff as well using https://godoc.org/golang.org/x/image/tiff#Encode ?

[mikecx]

I suppose we probably can. I was trying to make sure the file would be visible "in-browser" and generally tiff is not supported so it needs to be either jpg or png for that.

[willnorris]

Ah, good call. I didn't realize how poorly supported tiff was in browsers :)

[willnorris]

c1a9dab merged.