add withIcc command to override input file ICC profile

[mhirsch]

This PR adds a withIcc() command which overrides the input ICC profile.

I think the unit test is not quite sufficient, as the difference between the colorspace transformed image and the original input image are very subtle (the RGB values differ by 1 or 2 in most cases) with the ICC profile I'm using. fixtures.assertSimilar() will miss the difference if no transform occurred. The Bluish.icc file was taken from the lcms project via colord. I wonder if there's an ICC profile that would create a more dramatic difference? I hesitate to do something like an assert.strictEqual() on a couple of pixels of the image, as different systems might have different rounding errors during the colorspace conversion.

[coveralls]

Coverage increased (+0.009%) to 99.038% when pulling 59f2da9 on mhirsch:withIcc into 7c06a48 on lovell:master.

[lovell]

Hello, there's been some previous discussion in #218 about adding the ability to inject an ICC profile via the existing withMetadata operation. I'll attempt to update it with some thoughts on what the API might look like.

[lovell]

Hello, thank you for your time on this so far. I'm going to close this PR as it has drifted a long way behind the master branch - let's use #218 to track improvements.