Adding a new color preference setting that defines a custom color for underlining.

[rsmmr]

This is on top of the v3.1 branch. I believe I have followed most of the instructions in #286, except for two things:

• I had to modify [self.colorMap colorForKey:kColorMapUnderline] to account for the fact that we now expect it to return nil in this case. It would normally have returned a hardcoded red. Not so nice, maybe I'm missing something?
• I only added 1 one of the 3 test images, the non-retina version, as that's what I'm working on here. Not sure if/how I can create a retina version on a non-retina system? Also, I wasn't sure what exactly "run the test suite in Travis" means, so I left that part out. I would appreciate if you could add the missing images for testing when merging.

Please have a look. Feel free to send back to me if there's more to do.

[gnachman]

Good work! Very close to submittable; I just wrote a few small comments. I'll take care of adding the golden image for Travis.

[rsmmr]

Updated with your feedback, except for the change to result, see above.

[gnachman]

Merged. Thank you!

[rsmmr]

Thanks for merging and the feedback, and more generally for all your great work on iTerm2!

[josepheiba]

Hi, is there anyway I can change iTerm2 Colorspace from sRGB to P3 or to my current display's colorspace?

[gnachman]

@josepheiba The beta uses p3