Default to sRGB colors for CSI RGB values.

- By default consider CSI 24-bit RGB values to be display independent
  sRGB colors.
- Add an advanced option to disable this and consider the values to
  be calibrated to the given display that iTerm2 is running on.

Defaulting to interpret received CSI RGB values as sRGB makes more sense than the current method (that they are pre-calibrated to the display). Normally one is using iTerm to connect to a variety of hosts (also perhaps from a variety of hosts e.g., laptop, desktop, etc). These hosts want to send colors as sRGB and have them calibrated by the local host to the local display, rather than have to have a custom set of colors for each and every terminal (and their displays) that may connect to them.

I think it makes so much sense to use sRGB here that I'm not sure we even need a preference option or a choice, but since I'm changing the (as yet unreleased) behavior I thought I'd put it in there anyway.
I think you might be right. So the question is which is more efficient, checking the preference value when storing the character color, or checking the preference value when displaying the character color (with the understanding that there appears to be code to not do this if the color doesn't change). Or my preference, in order of preference: get rid of the sRGB preference option and always use sRGB, or save the state of this value at startup and never look at it again.